#include<iostream>
#include<map>
#include<vector>
#include <algorithm>
#include<math.h>
using namespace std;


int main()
{
    long long a, b, c, t;
    cin >> a >> b >> c;
    t = ceil((double)c*60*60/(a*60 - b));

    cout << ((a*60 <= b) ? -1 : t) << endl;
}