#include <cstdio>
#include <cstdlib>
#include <cstddef>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <iostream>
#include <iomanip>

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