#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main() {
  ll a, b, c;
  cin >> a >> b >> c;
  a *= 60;
  c *= 60 * 60;
  if (a < b) {
    puts("-1");
  } else if (a == b) {
    puts("0");
  } else {
    ll ans = ceil((double)c / (a - b));
    cout << ans << '\n';
  }
  return 0;
}