As i can see from my compilation error ( http://acm.timus.ru/ce.aspx?id=2270863 ), there is no System.Collections.Generic.HashSet in your .NET framework, so your version should be older than 3.5 (or you have disabled the HashSet).
Can you please say what exactly is supported and what is not; or which version of C#/.NET framework do you have?
Also, it would be great to include an example of minimal working code in the FAQ. For example, solution of problem #1000:
--
using System;

namespace Task1000a {
    class Program {
        static void Main(string[] args) {

            String[] numbers = Console.ReadLine().Split();
            Console.WriteLine((int.Parse(numbers[0]) + int.Parse(numbers[1])).ToString());
        }
    }
}
--

Thanks

Edited by author 11.10.2008 20:13

As far as i can see:
No Action<T1, T2, T3> supported (however, Action<T1> works);
No lambda-syntax like Action<T> = T arg => { /*do something*/ } supported; however, Action<T> = delegate(T arg) { /*do something*/ } works.

.NET 2.0 is supported now. We will move to .NET 3.5 in the near future.

Thanks.