#include <stdio.h>

int main() 
{
    printf("Hello world!\n"); //한 줄 주석
    // printf("Hello world!\n");
    printf("Hello world!\n");
    /* 여러 줄 주석 
    printf("Hello world!\n");
    printf("Hello world!\n");
    printf("Hello world!\n");
    printf("Hello world!\n"); */

    return 0;
}