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

int main(){
    int A,B;
    cin >> A >> B;
    for (int C=abs(A-B);C<=A+B;C++){
        if ((A+B)%C==0 and (B+C)%A==0 and (C+A)%B==0){
            cout << C << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}