#include <bits/stdc++.h>
using namespace std;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int a, b;
    cin >> a >> b;

    int ab = a + b;
    vector<bool> F(ab + 1, true);
    for(int i = 1; i <= (int)(sqrt(ab)); i++){
        if(F[i]){
            bool x = (a + b) % i == 0;
            bool y = (b + i) % a == 0;
            bool z = (i + a) % b == 0;
            if(x and y and z){
                cout << i << endl;
                return 0;
            }
            if(i == 1) continue;
            for(int j = i + i; j <= ab; j += i){
                F[j] = false;
            }
        }
    }

    cout << -1 << endl;

    return 0;
}