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

int main()
{
	long long A, B, C;
	cin >> A >> B >> C;

	long long ans = -1;
	if( A * 60 > B ) {
		long long d = A * 60 - B;
		ans = (C * 60 * 60 + d - 1) / d;
	}

	cout << ans << endl;
}