#include<bits/stdc++.h>
using namespace std;
using ll = long long;
int main(){
  int a, b;
  cin >> a >> b;
  if(a == 1 || b == 1){
    cout << 0 << "\n";
    return 0;
  }
  int c = max(a, b);
  int d = min(a, b);
  int r = c % d;
  while(r != 0){
    c = d;
    d = r;
    r = c % d;
  }
  if(d != 1){
    cout << -1 << "\n";
    return 0;
  }
  int ans = 0;
  vector<int> dp(100000, 0);
  dp[0] = 1;
  
  for (int i = 0; i <= a * b; i++){
    if(dp[i] == 1){
      dp[i + a] = 1;
      dp[i + b] = 1;
    }
  }
  for(int i=0; i<=a*b; i++){
    if(dp[i] == 0){
      ans++;
    }
  }
  cout << ans << "\n";

  return 0;
}