#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;
const double pi = 3.141592653589793238;

bool f(long long n) {
	while (n % 2 == 0) { n /= 2; }
	if (n == 1) { return 0; }
	return 1;
}

int main() {
	long long a, b, c;
	cin >> a >> b >> c;
	a *= 60, c *= 3600;
	if (a - b == 0) {
		cout << -1 << endl;
		return 0;
	}
	if ((c - 1) / (a - b) + 1 >= 0) { cout << (c - 1) / (a - b) + 1 << endl; }
	else { cout << -1 << endl; }
}