#include<iostream>
using namespace std;

int main(void) {
  long long a, b, c; cin >> a >> b >>c;
  long task;
  task=1;
  if(a*60 < b) {
    cout << -1 << endl;
    return 0;
    }
  while(true) {
    long hand_time;
    long code_time;
    hand_time = 60 * a * task;
    code_time = (3600 * c) + (b * task);
    if(hand_time > code_time) {
      cout << task << endl;
      return 0;
      }
    task++;
    }
  }