#include <bits/stdc++.h>
#define PI 3.14159265359
using namespace std;

int main() {
	int64_t A, B, C;
	cin >> A >> B >> C;

	A *= 60;
	C *= 3600;

	int64_t x = (C + (A - B) - 1) / (A - B);
	if (x < 0) x = -1;

	cout << x << endl;
}