using System;

namespace StudyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] inputs = Console.ReadLine().Trim().Split(' ');
            var a = Int64.Parse(inputs[0]) * 60;
            var b = Int64.Parse(inputs[1]);
            var c = Int64.Parse(inputs[2]) * 60 * 60;

            if (a - b <= 0)
                Console.WriteLine("-1");
            else
            {
                if (c % (a - b) == 0)
                    Console.WriteLine(c / (a - b));
                else
                    Console.WriteLine(c / (a - b) + 1);
            }
        }
    }
}