using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace YukiCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var va = Console.ReadLine().Split().Select(long.Parse).ToArray();
            long na = va[0] * 60 > va[1] ? (va[2] * 60 * 60 + va[0] * 60 - va[1] - 1) / (va[0] * 60 - va[1]) : -1;
            Console.WriteLine(na);
        }
    }
}