#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;

int main() {
  int a, b;
  cin >> a >> b;
  if (__gcd(a, b) != 1) {
    cout << -1 << endl;
    return 0;
  }

  vector<bool> ok(a * b + 1, 0);

  for (int i = 0; i <= b; i++) {
    for (int j = 0; j <= a; j++) {
      int n = a * i + b * j;
      if (n <= a * b) ok[n] = 1;
    }
  }

  int res = 0;

  for (int i = 0; i <= a * b; i++) if (!ok[i]) res++;
  cout << res << endl; 
  return 0;
}