#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int A, B, C;
	cin >> A >> B >> C;
	A = 60 * A;
	C = 60 * 60 * C;
	if (A <= B) {
		cout << -1 << endl;
		return 0;
	}
	//xA >= Bx + C
	int x = max((int)1, (C / (A - B)) - 3);
	while (x*A < B*x + C) {
		x++;
	}
	cout << x << endl;
}