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

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

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

    int x = a + b;

    vector<int> A;
    for(int i = 1; i <= (int)(sqrt(x)); i++){
        if(x % i == 0){
            A.push_back(i);
            A.push_back(x / i);
        }
    }

    sort(A.begin(), A.end());

    for(int i = 0; i < A.size(); i++){
        if(A[i] != a and A[i] != b){
            bool l = (a + b) % A[i] == 0;
            bool m = (b + A[i]) % a == 0;
            bool n = (A[i] + a) % b == 0;
            if(l and m and n){
                cout << A[i] << endl;
                return 0;
            }
        }
    }

    cout << -1 << endl;

    return 0;
}