#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>

using namespace std;


int main() {
	int  i, j, k;
	long long a, b, c;

	cin >> a >> b >> c;

	a *= 60;
	c *= 3600;

	if (a <= b) {
		cout << -1 << endl;
		//getchar();
		//getchar();
		return 0;
	}

	long long x;
	long long sa = a - b;


	x = c / a;



	long long yo = c % a;

	long long ama = x * b;

	x = x + (x / sa);

	c += (x / sa) * b;
	c += ama;

	while(true){

		//if (c <= a*x) {
		if (c >= a*x) {
			c += b;
			x++;
		}
		else {
			break;
		}

	}

	/*
	long long num = 0;

	while (true) {

		if (yo > b * num) {
			num++;
	
		}
		else {
			break;
		}

	}
	*/

	//cout << x + num << endl;
	cout << x << endl;





	getchar();
	getchar();
	return 0;
}