You are given a graph with $n$ nodes and $m$ directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is $3$. Your task is find a path whose value is the largest.
The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), denoting that the graph has $n$ nodes and $m$ directed edges.

The second line contains a string $s$ with only lowercase English letters. The $i$-th character is the letter assigned to the $i$-th node.

Then $m$ lines follow. Each line contains two integers $x, y$ ($1 \leq x, y \leq n$), describing a directed edge from $x$ to $y$. Note that $x$ can be equal to $y$ and there can be multiple edges between $x$ and $y$. Also the graph can be not connected.
In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
import sys
def ache(arr_n,arr_m,s,arr_x,arr_y):
    if s in arr_m:
        return -1
    if arr_y[s-1] not in arr_x:
        arr_x[arr_y[s-1]]=1
    else:
        arr_x[arr_y[s-1]]=arr_x[arr_y[s-1]]+1
    if s in arr_n:
        ans=0
        for each in arr_n[s]:
            p=ache(arr_n,arr_m.union({s}),each,arr_x.copy(),arr_y)
            if p==-1:
                return p
            else:
                ans=max(p,ans)
        return ans
    else:
        c=0
        for each in arr_x:
            c=max(c,arr_x[each])
        return c
def beauty(n,m,s,x,y):
    w={}
    for i in range(m):
        if x[i] in w:
            w[x[i]].append(y[i])
        else:
            w[x[i]]=[y[i]]
    flag=False
    cm=0
    for key in w:
        o=ache(w,set(),key,{},s)
        if o==-1:
            flag=True
            break
        cm=max(cm,o)
    if flag:
        return -1
    return cm
x=sys.stdin.readline().split()
n=int(x[0])
m=int(x[1])
s=sys.stdin.readline()
x,y=[],[]
for i in range(m):
    k=sys.stdin.readline().split()
    x.append(int(k[0]))
    y.append(int(k[1]))
print(beauty(n,m,s,x,y))