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

int main()
{
	ll A, B, C;
	cin >> A >> B >> C;
	A *= 60;
	C *= 60 * 60;
	if (B < A) {
		cout << ((C + A - B - 1) / (A - B)) << endl;
	}
	else {
		cout << -1 << endl;
	}
	return 0;
}