prob7.cs

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

class Prime
{
  private static List<int> odd_primes = new List<int>();

  private static bool IsPrime(int val)
  {
    foreach (var i in odd_primes)
      {
	if (i*i > val)
	  {
	    break;
	  }
	if (val % i == 0)
	  {
	    return false;
	  }
      }
    return true;
  }

  public static IEnumerable<int> seq()
  {
    yield return 2;
    for (var i = 3; ; i += 2)
      {
	if (IsPrime(i))
	  {
	    odd_primes.Add(i);
	    yield return i;
	  }
      }
  }
}

class Prob7
{
  static void Main()
  {
    Console.WriteLine(Prime.seq().Skip(10000).First());
  }
}