#include<iostream>
using namespace std;

int main() {
	long long a, b, c;
	cin >> a >> b >> c;
	if (a < b) {
		cout << -1 << endl;
		return 0;
	}
	else {
		long long ans = 1;
		while (1) {
			if (a * 60 * ans> c * 3600 + ans * b) {
				cout << ans << endl;
				return 0;
			}
			ans++;
		}
	}
}