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 ab = a-b;
        long ans = -1;
        if(a>b){
            if(c%ab==0){
                long aa = c/ab;
                if(aa>0){
                    ans = aa;
                }
            }else{
                long aa = (c/ab)+1;
                ans = aa;
            }
        }
        WriteLine(ans);
    }
}