#include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;

const double PI = 3.1415926535897932384626433832795;

const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { -1, 0, 1, 0 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	ll a, b, c;
	cin >> a >> b >> c;

	if (60 * a <= b) {
		cout << -1 << endl;
	} else {
		ll h = 3600 * c;
		cout << (ll)(ceil((ld)h / (60 * a - b))) << endl;
	}

	// 作成時間
	// /
	// 手作業時間 - 自動作業時間
	return 0;
}