#include<iostream>
#include<math.h>

using namespace std;

int main() {

	long long a, b, c;
	double x = 0;
	cin >> a >> b >> c;
	x = (-pow(60, 2)*c) / (b - 60 * a);
	long long ans = ceil(x);
	if (x < 0) {
		cout << -1;
		return 0;
	}
	
	cout << ans;

	return 0;
}