using System;
using System.Linq;

class No734
{
    static void Main()
    {
        var num = Console.ReadLine().Split(' ').Select(x => Int64.Parse(x)).ToArray();
        var a = num[0] * 60;
        var b = num[1];
        var c = num[2] * 3600;
        long ans = a < b ? -1 : (c-1)/(a-b)+1;
        Console.WriteLine(ans);
    }
}