#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef long long ll;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	ll a, b, c;
	cin >> a >> b >> c;
	//seconds-
	a *= 60;
	c *= 3600;

	if (a > b) {
		ll ans = c / (a - b) + 1;
		if (c % (a - b) == 0) {
			cout << ans - 1 << endl;
		}
		else {
			cout << ans << endl;
		}
	}
	else {
		cout << -1 << endl;
	}
	return 0;
}