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

int main()
{
	int64_t a, b, c;
	cin >> a >> b >> c;
	a *= 60;
	c *= 3600;
	if (a <= b)
	{
		cout << -1 << endl;
		return 0;
	}
	cout << (c + a - b - 1) / (a - b) << endl;
	return 0;
}