#include <iostream>
using namespace std;
int main(){
	long long A, B, C;
	cin >> A >> B >> C;
	if (A * 60 <= B){
		cout << -1 << endl;
	} else {
		cout << (C * 3600 + A * 60 - B - 1) / (A * 60 - B) << endl;
	}
}