using System;

using System.Threading;

using System.Threading.Tasks;

public class Program

{

    public static async Task Main()

    {

        Console.WriteLine("1");

        var task = UsuallySync();

        Console.WriteLine("3");

​

        await task;

    }

    static SemaphoreSlim _lock = new SemaphoreSlim(1);

    static async Task UsuallySync()

    {

        await _lock.WaitAsync();

        Console.WriteLine("2");

    }

}