#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  ll a, b, c;
  cin >> a >> b >> c;
  a *= 60;
  c *= 3600;
  if (a - b <= 0) {
    cout << -1 << endl;
  } else {
    cout << ((c + a - b - 1) / (a - b)) << endl;
  }
  return 0;
}