#include <bits/stdc++.h>
#define ll long long
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

using namespace std;
int main() {
  INIT;
  
  int a,b;
  cin >> a >> b;
  
  for (size_t c = b-a; c < 10^18; c+=b) {
    if((a+b) % c == 0 && (c+b) % a == 0 && (a+c) % b == 0){
      //cout << c << endl;
      printf("%d", c);
      exit(0);
    }
  }
  //cout << "-1" << endl;
  printf("-1");
  
  return 0;
}