#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;





int main()
{

    int Q;
    Q = 1;
    //cin >> Q;


    while (Q--) {
       
        ll a, b, c;
        cin >> a >> b >> c;
        a *= 60;
        c *= 3600;
        if (a <= b) {
            cout << -1;
            return 0;
        }
        ll x = c / a;
        ll y = a - b;
        ll ans = c / y;
        if (c % y)ans++;
        cout << ans;
    }
    return 0;
}