using System;
using static System.Console;
using static System.Math;

public class Hello{
    public static void Main(){
        string[] input = ReadLine().Split(' ');
        long a = long.Parse(input[0]);
        long b = long.Parse(input[1]);
        long c = long.Parse(input[2]);
        a *= 60;
        c *= 3600;
        long ans = -1;
        if(a>b){
            if(c%(a-b)==0){
                long aa = c/(a-b);
                ans = aa;
            }else{
                long aa = 1+c/(a-b);
                ans = aa;
            }
        }
        WriteLine(ans);
    }
}