#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define fi first
#define se second
int main(){
    int a,b;
    cin >> a >> b;
    for(int c=1;c < a+b;c++)
    {
        if((a+b)%c == (b+c)%a && (c+a)%b == (a+b)%c && (b+c)%a==0){
            cout << c << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}