#include <iostream>
#include<cmath>
using namespace std;
int main() {
	long long a, b, c;
	cin >> a >> b >> c;
	a *= 60;
	c *= 3600;
	if (a < b) {
		cout << -1 << endl;
		return 0;
	}
	cout << ceil(((double)c / (b - a) * -1));
}