#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<string>
#include<iostream>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include <sstream>
#include<iomanip>
#include <ctype.h>
using namespace std;

//#include<bits/stdc++.h>


int main() {

	long long int a, b, c;
	cin >> a >> b >> c;
	a *= 60;
	c *= 3600;
	
	//cout << c;
	//cout << a << " " << c << endl;;
	long long int r = c/(a-b);
	if (c%a)r++;
	if (a <= b)cout << -1 << endl;
	else cout << r++ << endl;
	

	return 0;
}