using System;

using System.Linq;

public class Program

{

    public static void Main()

    {       

        var n = 100000;

        var a = Enumerable.Range(0,n+1).Select(_ => true).ToArray();

​

        for(var i=2;i<Math.Sqrt(n);i++)

        {

            if(a[i])

            {

                for(var j = i*i;j<=n;j += i)

                {

                    a[j] = false;

                }

            }

        }

        var result = a.Select( (x,i) => new {IsPrime = x,Prime = i}).Where(x => x.IsPrime && x.Prime > 1).Sum(x => x.Prime);

        Console.WriteLine(result);

    }

    private bool[] MakeSieve(int max)

    {

        // Make an array indicating whether numbers are prime.

        bool[] is_prime = new bool[max + 1];

        for (int i = 2; i <= max; i++) is_prime[i] = true;

​

        // Cross out multiples.

        for (int i = 2; i <= max; i++)

        {

            // See if i is prime.

            if (is_prime[i])

            {

                // Knock out multiples of i.

                for (int j = i * 2; j <= max; j += i)

                    is_prime[j] = false;

            }

        }

        return is_prime;

    }

}