You are given a matrix consisting of digits zero and one, its size is *n* × *m*. You are allowed to rearrange its rows. What is the maximum area of the submatrix that only consists of ones and can be obtained in the given problem by the described operations?

Let's assume that the rows of matrix *a* are numbered from 1 to *n* from top to bottom and the columns are numbered from 1 to *m* from left to right. A matrix cell on the intersection of the *i*-th row and the *j*-th column can be represented as (*i*, *j*). Formally, a submatrix of matrix *a* is a group of four integers *d*, *u*, *l*, *r* (1 ≤ *d* ≤ *u* ≤ *n*; 1 ≤ *l* ≤ *r* ≤ *m*). We will assume that the submatrix contains cells (*i*, *j*) (*d* ≤ *i* ≤ *u*; *l* ≤ *j* ≤ *r*). The area of the submatrix is the number of cells it contains.
The first line contains two integers *n* and *m* (1 ≤ *n*, *m* ≤ 5000). Next *n* lines contain *m* characters each — matrix *a*. Matrix *a* only contains characters: "0" and "1". Note that the elements of the matrix follow without any spaces in the lines.

from bisect import bisect,bisect_left

from collections import *
from math import gcd,ceil,sqrt,floor,inf
from heapq import *
from itertools import *
from operator import add,mul,sub,xor,truediv,floordiv
from functools import *

#------------------------------------------------------------------------
import os
import sys

from io import BytesIO, IOBase
# region fastio
 
BUFSIZE = 8192
 
class FastIO(IOBase):
    newlines = 0
 
    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None
 
    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()
 
    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()
 
    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)
 
 
class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")
 
 
sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")
 

#------------------------------------------------------------------------
def RL(): return map(int, sys.stdin.readline().split())
def RLL(): return list(map(int, sys.stdin.readline().split()))
def N(): return int(input())
def A(n):return [0]*n
def AI(n,x): return [x]*n
def A2(n,m): return [[0]*m for i in range(n)]
def G(n): return [[] for i in range(n)]
def GP(it): return [[ch,len(list(g))] for ch,g in groupby(it)]
#------------------------------------------------------------------------


from types import GeneratorType
 
 
def bootstrap(f, stack=[]):
    def wrappedfunc(*args, **kwargs):
        if stack:
            return f(*args, **kwargs)
        else:
            to = f(*args, **kwargs)
            while True:
                if type(to) is GeneratorType:
                    stack.append(to)
                    to = next(to)
                else:
                    stack.pop()
                    if not stack:
                        break
                    to = stack[-1].send(to)
            return to
 
    return wrappedfunc

mod=10**9+7
farr=[1]
ifa=[]
 
def fact(x,mod=0):
    if mod:
        while x>=len(farr):
            farr.append(farr[-1]*len(farr)%mod)
    else:
        while x>=len(farr):
            farr.append(farr[-1]*len(farr))
    return farr[x]
 
def ifact(x,mod):
    global ifa
    fact(x,mod)
    ifa.append(pow(farr[-1],mod-2,mod))
    for i in range(x,0,-1):
        ifa.append(ifa[-1]*i%mod)
    ifa.reverse()
 
def per(i,j,mod=0):
    if i<j: return 0
    if not mod:
        return fact(i)//fact(i-j)
    return farr[i]*ifa[i-j]%mod
    
def com(i,j,mod=0):
    if i<j: return 0
    if not mod:        
        return per(i,j)//fact(j)
    return per(i,j,mod)*ifa[j]%mod
 
def catalan(n):
    return com(2*n,n)//(n+1)
 
def isprime(n):
    for i in range(2,int(n**0.5)+1):
        if n%i==0:
            return False
    return True

def floorsum(a,b,c,n):#sum((a*i+b)//c for i in range(n+1))
    if a==0:return b//c*(n+1)
    if a>=c or b>=c: return floorsum(a%c,b%c,c,n)+b//c*(n+1)+a//c*n*(n+1)//2
    m=(a*n+b)//c
    return n*m-floorsum(c,c-b-1,a,m-1)

def inverse(a,m):
    a%=m
    if a<=1: return a
    return ((1-inverse(m,a)*m)//a)%m
 
def lowbit(n):
    return n&-n
 
class BIT:
    def __init__(self,arr):
        self.arr=arr
        self.n=len(arr)-1
        
    def update(self,x,v):
        while x<=self.n:
            self.arr[x]+=v
            x+=x&-x
 
    def query(self,x):
        ans=0
        while x:
            ans+=self.arr[x]
            x&=x-1
        return ans

class ST:
    def __init__(self,arr):#n!=0
        n=len(arr)
        mx=n.bit_length()#取不到
        self.st=[[0]*mx for i in range(n)]
        for i in range(n):
            self.st[i][0]=arr[i]
        for j in range(1,mx):
            for i in range(n-(1<<j)+1):
                self.st[i][j]=max(self.st[i][j-1],self.st[i+(1<<j-1)][j-1])
    def query(self,l,r):
        if l>r:return -inf
        s=(r+1-l).bit_length()-1
        return max(self.st[l][s],self.st[r-(1<<s)+1][s])
        
class DSU:#容量+路径压缩
    def __init__(self,n):
        self.c=[-1]*n
 
    def same(self,x,y):
        return self.find(x)==self.find(y)
 
    def find(self,x):
        if self.c[x]<0:
            return x
        self.c[x]=self.find(self.c[x])
        return self.c[x]
 
    def union(self,u,v):
        u,v=self.find(u),self.find(v)
        if u==v:
            return False
        if self.c[u]>self.c[v]:
            u,v=v,u
        self.c[u]+=self.c[v]
        self.c[v]=u
        return True
 
    def size(self,x): return -self.c[self.find(x)]
    
class UFS:#秩+路径
    def __init__(self,n):
        self.parent=[i for i in range(n)]
        self.ranks=[0]*n
 
    def find(self,x):
        if x!=self.parent[x]:
            self.parent[x]=self.find(self.parent[x])
        return self.parent[x]
 
    def union(self,u,v):
        pu,pv=self.find(u),self.find(v)
        if pu==pv:
            return False
        if self.ranks[pu]>=self.ranks[pv]:
            self.parent[pv]=pu
            if self.ranks[pv]==self.ranks[pu]:
                self.ranks[pu]+=1
        else:
            self.parent[pu]=pv
 
def Prime(n):
    c=0
    prime=[]
    flag=[0]*(n+1) 
    for i in range(2,n+1):
        if not flag[i]:
            prime.append(i)
            c+=1
        for j in range(c):
            if i*prime[j]>n: break
            flag[i*prime[j]]=prime[j]
            if i%prime[j]==0: break
    return flag
 
def dij(s,graph):
    d={}
    d[s]=0
    heap=[(0,s)]
    seen=set()
    while heap:
        dis,u=heappop(heap)
        if u in seen:
            continue
        seen.add(u)
        for v,w in graph[u]:
            if v not in d or d[v]>d[u]+w:
                d[v]=d[u]+w
                heappush(heap,(d[v],v))
    return d

def bell(s,g):#bellman-Ford
    dis=AI(n,inf)
    dis[s]=0
    for i in range(n-1):
        for u,v,w in edge:
            if dis[v]>dis[u]+w:
                dis[v]=dis[u]+w
    change=A(n)
    for i in range(n):
        for u,v,w in edge:
            if dis[v]>dis[u]+w:
                dis[v]=dis[u]+w
                change[v]=1
    return dis

def lcm(a,b): return a*b//gcd(a,b)
def lis(nums):
    res=[]
    for k in nums:
        i=bisect.bisect_left(res,k)
        if i==len(res):
            res.append(k)
        else:
            res[i]=k
    return len(res)

def RP(nums):#逆序对
    n = len(nums)
    s=set(nums)
    d={}
    for i,k in enumerate(sorted(s),1):
        d[k]=i
    bi=BIT([0]*(len(s)+1))
    ans=0
    for i in range(n-1,-1,-1):
        ans+=bi.query(d[nums[i]]-1)
        bi.update(d[nums[i]],1)
    return ans
 
class DLN:
    def __init__(self,val):
        self.val=val
        self.pre=None
        self.next=None

def nb(i,j,n,m):
    for ni,nj in [[i+1,j],[i-1,j],[i,j-1],[i,j+1]]:
        if 0<=ni<n and 0<=nj<m:
            yield ni,nj

def topo(n):
    q=deque()
    res=[]
    for i in range(1,n+1):
        if ind[i]==0:
            q.append(i)
            res.append(i)
    while q:
        u=q.popleft()
        for v in g[u]:
            ind[v]-=1
            if ind[v]==0:
                q.append(v)
                res.append(v)
    return res

@bootstrap
def gdfs(r,p):
    for ch in g[r]:
        if ch!=p:
            yield gdfs(ch,r)
    yield None



t=1
for i in range(t):
    n,m=RL()
    res=[]
    for i in range(n):
        res.append(input())
    dp=A2(n,m)
    for i in range(n):
        dp[i][0]=int(res[i][0]=='1')
        for j in range(1,m):
            if res[i][j]=='1':
                dp[i][j]=dp[i][j-1]+1
    ans=0
    #print(dp)
    for j in range(m):
        cur=[]
        for i in range(n):
            cur.append(int(res[i][j]))
        #print(cur)
        cur.sort(reverse=True)
        for i,x in enumerate(cur,1):
            ans=max(i*x,ans)
    
    

    
    print(ans)
                
                
                    
    
    
''' 
sys.setrecursionlimit(200000)
import threading
threading.stack_size(10**8)
t=threading.Thr
ead(target=main)
t.start()
t.join()
'''

