where to start, from where the end

当进入一个全新世界的时候，面对世外桃源，我们总会不由自主地喊了一句 Hello world !

#include <stdio.h>
#include <stdlib.h>
int main (int argc, char **argv) {
    printf("%s\n", "Hello world");
    exit(0);
}

当是时候离开这个世外桃源的时候，我们也得礼貌地前后呼应 Goodbye world !

#include <stdio.h>
#include <stdlib.h>
int main (int argc, char **argv) {
    printf("%s\n", "Goodbye world");
    exit(0);
}

where to start, from where the end