using System;

using System.Threading.Tasks;

using System.Collections.Generic;

public class Program

{

    public static async Task Main()

    {

        var enumeration = AsyncEnumeration(-1);

        await foreach(int x in enumeration)

        {

            Console.WriteLine(x);

        }

    }

    public static IAsyncEnumerable<int> AsyncEnumeration(int count)

    {

        if (count < 1)

            throw new ArgumentOutOfRangeException();

        return AsyncEnumerationCore(count);

    }

    private static async IAsyncEnumerable<int> AsyncEnumerationCore(int count)

    {

        for(int i = 0; i < count; i++)

        {

            yield return i;

            await Task.Delay(1);

        }

    }

}