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

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0); 
	
	long long a, b, c;
	cin >> a >> b >> c;
	a *= 60;
	c *= 60 * 60;
	if(a - b == 0) {
		cout << -1 << endl;
		return 0;
	}
	long long ans = (c - 1) / (a - b) + 1;
	cout << (ans >= 0 ? ans : -1) << endl;
	return 0;
}