/* -*- coding: utf-8 -*-
 *
 * 734.cc:  No.734 Careful Engineer - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

typedef long long ll;

/* global variables */

/* subroutines */

/* main */

int main() {
  ll a, b, c;
  scanf("%lld%lld%lld", &a, &b, &c);

  // 60a*x >= b*x+3600c -> x >= 3600c / (60a-b)

  ll d = 60 * a - b;
  if (d <= 0) puts("-1");
  else {
    ll x = (3600 * c + d - 1) / d;
    printf("%lld\n", x);
  }
  return 0;
}