using System;

namespace Yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var line = Console.ReadLine();
            int age = int.Parse(line);
            if (age - 7 * 2 >= 0)
            {
                Console.WriteLine(age - 7);
            }
            else
            {
                Console.WriteLine(-1);
            }

        }
    }
}