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

int cnt[10005];

int main(){
  int a, b; cin >> a >> b;
  if(gcd(a,b)>1){
    cout << -1 << endl;
    return 0;
  }
  int m = a*b-a-b;
  for(int i = 0; i <= 100; i++){
    for(int j = 0; j <= 100; j++){
      cnt[a*i+b*j]++;
    }
  }
  int ans = 0;
  for(int i = 1; i <= m; i++){
    if(cnt[i]==0) ans++;
  }
  cout << ans << endl;
  return 0;
}