using System;

public class Program

{

    static void HoanVi(ref int a, ref int b)

    {

        int temp = a;

        a = b;

        b = temp;

    }

    public static void Main()

    {

        int a, b;

        Console.WriteLine("Nhap a : ");

        a = int.Parse(Console.ReadLine());

        Console.WriteLine("Nhap b: ");

        b = int.Parse(Console.ReadLine());

        Console.WriteLine("a truoc khi hoan vi la: {0}", a);

        Console.WriteLine("b truoc khi hoan vi la: {0}", b);

        Console.WriteLine("-----------------------------------------");

        HoanVi(ref a, ref b);

        Console.WriteLine("a sau khi hoan vi la: {0}", a);

        Console.WriteLine("b sau khi hoan vi la: {0}", b);

​

    }

}