#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() {
  int n, m;
  cin >> n >> m;
  if (n <= m) {
    cout << 1 << endl;
  }
  else if (n > m && n % 2) {
    cout << -1 << endl;
  }
  else if (n / 2 > m) {
    cout << -1 << endl;
  }
  else {
    cout << 2 << endl;
  }
  return 0;
}