using System;

namespace yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {

            string[] str = Console.ReadLine().Split(' ');
            int a = int.Parse(str[0]);
            int b = int.Parse(str[1]);
            int num = 0;
            if (a - b == b)
            {

            }
            else
            {
                num = (a / 2) - 5;
            }
            Console.WriteLine(num);
        }
    }
}