using System;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("M_1");

        var t = f1();

        Console.WriteLine("M_2");

        t.Wait();

        Console.WriteLine("M_3");

    }

    static async Task f1()

    {

        Console.WriteLine("f1_1");

        await f2();

        Console.WriteLine("f1_2");

    }

    static async Task f2()

    {

        Console.WriteLine("f2_1");

        await Task.Delay(TimeSpan.FromSeconds(3));

        Console.WriteLine("f2_2");

    }

​

}