#include <bits/stdc++.h>
#define int long long
#define double long double
#define INF 1e18
using namespace std;
signed main() {
  int A, B;
  cin >> A >> B;
  int ans = ((2*A+B)%B == 0 && (A+2*B)%A == 0) ? A+B : -1;
  for (int K = 2; K*K <= A+B; K++) {
    if ((A+B)%K == 0) {
      int C = (A+B)/K;
      if ((A+C)%B == 0 && (B+C)%A == 0 && C != A && C != B) {
        ans = C;
      }
    }
  }
  cout << ans << endl;
}