#include <bits/stdc++.h>
using namespace std;
int main(){
  long long a, b, c; cin >> a >> b >> c;
  a *= 60; c *= 3600;
  if (a - b > 0) {
    cout << (c + (a-b-1)) / (a - b) << endl;
  } else cout << -1 << endl;
  return 0;
}