D: Is greed the best?

story

In Japan, where there are 1, 5, 10, 50, 100, 500 yen coins, it is known that the number of coins can be minimized by using as many coins as possible when paying a certain amount. ..

If the amount of coins is different from that of Japan, it is not always possible to minimize it by paying greedily.

What are the conditions that the amount of coins must meet in order to be optimally paid greedily?

problem

TAB was curious about the above, so I decided to first consider the case where there are only three types of coins, 1, A and B.

Since A and B are given, output the smallest amount of money that will not minimize the number of sheets if you pay greedily.

Also, if the greedy algorithm is optimal for any amount of money, output -1.

Input format


A B

Constraint

* 1 <A \ leq 10 ^ 5
* A <B \ leq 10 ^ 9



Input example 1


4 6

Output example 1


8

If you greedily pay 8 yen, you will pay 6 + 1 \ times 2 for a total of 3 cards, but you can pay 4 \ times 2 for a total of 2 cards.

Input example 2


2 1000000000

Output example 2


-1

It is best to greedily pay any amount.





Example

Input

4 6


Output

8
#pragma GCC optimize("O2")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdarg>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#define whlie while
#define retunr return
#define reutrn return
#define reutnr return
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define inf 1001001001
#define infLL (1LL << 61)
#define MOD 1000000007
#define FOR(i,a,b) for(int (i)=(int)(a); (i)<(int)(b); (i)++) // [a,b)
#define rep(i,N) FOR((i), 0, (int)(N)) // [0,N)
#define FORR(i,a,b) for(int (i)=(int)(b) - 1; (i)>=(int)(a); (i)--)
#define repr(i,N) FORR((i), 0, (int)(N))
#define each(x,v) for(auto &x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define vrep(v,it) for(auto it=v.begin();it!=v.end();it++)
#define vrepr(v,it) for(auto it=v.rbegin();it!=v.rend();it++)
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define inc(...) char __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
#define ind(...) double __VA_ARGS__; in(__VA_ARGS__)
#define inpii(...) pii __VA_ARGS__; in(__VA_ARGS__)
#define rand mtrand
#define randinit random_device seed_gen; mt19937 mtrand(seed_gen()+2)

#ifdef LOCAL  
  #define trc(...) do { cout << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define stopif(val) assert( !(val) )
  #define trcv(v,...) do { cout << #v << " = "; vector_debug(v , ##__VA_ARGS__);cout << endl;} while(0)
#else
  #define trc(...)
  #define stopif(...)
  #define trcv(...)
#endif

using namespace std;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vpii = vector<pii>;
using vvi = vector< vector<int> >;

struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(15);} } iosetupnya;
/* 
int gcd(int a, int b){if(a>b) swap(a,b); return a==0 ? b : gcd(b%a,a);} ll gcd(ll a, ll b){if(a>b) swap(a,b); return a==0 ? b : gcd(b%a,a);}
ll lcm(int a, int b){return (1LL * a / gcd(a,b)) * b;} ll lcm(ll a, ll b){return (a / gcd(a,b)) * b;}
*/
inline ll pow(int a, int b){ll ans = 1; rep(i,b) ans *= a; return ans;} inline ll pow(ll a, ll b){ll ans = 1; rep(i,b) ans*= a; return ans;}
inline ll pow(int a, ll b){ll ans = 1; rep(i,b) ans *= a; return ans;} inline ll pow(ll a, int b){ll ans = 1; rep(i,b) ans*= a; return ans;}
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename C> inline void _cin(C &c){cin >> c;}
template<typename T,typename U> inline void _cin(pair<T,U> &p){cin >> p.fi; cin >> p.se;}
template<typename C> inline void _cout(const C &c){cout << c;}
template<typename T,typename U> inline void _cout(const pair<T,U> &p){cout << p.fi << ' ' << p.se;}
void in(){} template <typename T,class... U> void in(T &t,U &...u){ _cin(t); in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,U ...u){ _cout(t); if(sizeof...(u)) cout << ' '; out(u...);}
template<typename C> inline void in(vector<C> &v,int N=-1){if(sz(v) != 0){int M=(N == -1) ? sz(v) : N; rep(i,M) _cin(v[i]);}else{C c;rep(i,N) v.pb((_cin(c),c));}}
template<typename C> inline void in(C v[],int N){rep(i,N) _cin(v[i]);}
template<typename C> inline void out(const vector<C> &v,int N=-1){int M=(N == -1) ? sz(v) : N; rep(i,M) {cout<<( (i)?" ":"" ); _cout(v[i]);} cout<<"\n";}
template<typename C> inline void out(C v[],int N){rep(i,N) {cout<<( (i)?" ":"" ); _cout(v[i]);} cout<<"\n";}
template<typename C> inline void vector_debug(const vector<C> &v,int N=-1){cout << "{"; int M=(N == -1) ? sz(v) : N; rep(i,M) {cout<<( (i)?", ":"" ); _cout(v[i]);} cout<<"}";}
template<typename C> inline void vector_debug(C v[], int N){cout << "{"; rep(i,N) {cout<<((i)?", ":""); _cout(*(v+i));} cout<<"}";}
void dbg_out(){cout << endl;} template <typename T,class... U> void dbg_out(const T &t,U ...u){ _cout(t); if(sizeof...(u)) cout << ", "; dbg_out(u...);}
template<typename C,class... U> void dbg_out(const vector<C> &v,U ...u){vector_debug(v); if(sizeof...(u)) cout << ", "; dbg_out(u...);}
template<typename C,class... U> void dbg_out(const vector<vector<C>> &v,U ...u){cout << "{ "; rep(i,sz(v)) {if(i)cout<<", "; vector_debug(v[i]);} cout << " }"; if(sizeof...(u)) cout << ", "; dbg_out(u...);}
template<typename C> inline C vmax(const vector<C> &v){C n=v[0]; rep(i,sz(v)) amax(n,v[i]); return n;} template<typename C> inline C vmax(C v[], int N){C n=v[0]; rep( i , N ) amax(n,v[i]); return n;}
template<typename C> inline C vmin(const vector<C> &v){C n=v[0]; rep(i,sz(v)) amin(n,v[i]); return n;} template<typename C> inline C vmin(C v[], int N){C n=v[0]; rep( i , N ) amin(n,v[i]); return n;}
void die(string s){out(s); exit(0);}

////////////
/// main ///
////////////

int main(){
  ini(A,B);
  if(B % A == 0) die("-1");
  // A * n > Bなる最小のnを考える
  int n = B / A + 1;
  if(n < A * n - B + 1) out(n * A);
  else out(-1);
}
