using System;

using System.Collections.Generic;

​

public class Sequence<T> : List<T>{

    public Sequence<T> Swap(){      

        for (var i = 0; i < Count-1; i+=2){

            var temp = this[i];

            this[i] = this[i+1];

            this[i + 1] = temp;

        }

        return this;

    }

}

​

public class Program

{   

    public static void Main()

    {

        Sequence<string> s = new Sequence<string>();

        Console.WriteLine("Input:");

        var input = Console.ReadLine();

        foreach (var elem in input.Split(' '))

            s.Add(elem);

        s.Swap();

        Console.WriteLine("Output: ");

        foreach (var elem in s)

            Console.Write(elem + " ");

    }

}