Correct Input

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    int x;
    char buf[BUFSIZ];
    int first = 0;
    printf("Input: ");
    while (1) {
        if (first) {
            printf("Re-input: ");
        }
        first = 1;
        fgets(buf, BUFSIZ, stdin);
        x = atoi(buf);
        if (100 <= x && x <= 500) break;
    }
    printf("input = %d\n", x);
    return 0;
}