#include <stdio.h>
#include <stdlib.h>
Bai1()
{
    printf ("Hello World");
}
Bai2()
{
    printf ("Goodbye");
}
debai()
{
    int a;
    printf ("Ban chon bai: ");
    scanf ("%d", &a);
    switch (a)
    {
        case 1:
            {
                Bai1();
                break;
            }
        case 2:
            {
                Bai2();
                break;
            }
        case 3:
            {
                exit(0);
            }
        default:
            printf ("Ban nhap sai");
            break;
    }
}
int main ()
{
        debai();
        return 0;
}