using System;
using System.Linq;
class Program
{
    static void Main()
    {

        long[] numbers = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        long maxNumber = numbers.Max(); //Gets the max number of the array
        long totalSumOfNumber = numbers.Sum(); //Gets the total sum of the numbers in the array
        long check = (totalSumOfNumber - maxNumber) / 2; //Gets the sum of the numbers int the array without the max number

        if (check == maxNumber) //Checks if the sum is equal to the max number
        {
            Console.WriteLine("{0}", check);
        }
        else
        {

            if (numbers.Length == 1)
            {
                Console.WriteLine(0);
            }
            else
            {
                Console.WriteLine("no");
            }
        }
    }
}