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;
        int ans = -1;
        if(a>b){
            for(int i=1;i<c;i++){
                long te = a*i;
                long pr = c+b*i;
                if(te>pr){
                    ans = i;
                    break;
                }
            }
        }
        WriteLine(ans);
    }
}