#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int a, b;
  cin >> a >> b;
  if (gcd(a, b) != 1) {
    cout << -1 << '\n';
  }
  else {
    set<int> s;
    rep(x, a * b) s.insert(x);
    for (int i = 0; i <= b; ++i) {
      for (int j = 0; j <= a; ++j) {
        s.erase(i * a + j * b);
      }
    }
    cout << s.size() << '\n';
  }
  return 0;
}