#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define int long long
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e18;

signed main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int a, b;
    cin >> a >> b;
    int c = INF;
    for(int i = 1; i * i <= a + b; i++){
        if((a + b) % i == 0){
            if((a + i) % b == 0 && (i + b) % a == 0 && i != a && i != b) c = min(c, i);
            int j = (a + b) / i;
            if((a + j) % b == 0 && (j + b) % a == 0 && j != a && j != b) c = min(c, j);
        }
    }
    if(c == INF) cout << -1 << endl;
    else cout << c << endl;
}