using System;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        int age = int.Parse(Console.ReadLine());
        age -= 7;

        if (age < 7)
        {
            Console.WriteLine("-1");
        }
        else
        {
            Console.WriteLine(age);
        }

        Console.Read();
    }
}