/*
 * Program to "count down" from n to 0.
 */
#include <stdio.h>

void countdown(int n);

int main(void) {

	int n;
	printf("enter start number\n");
	if (scanf("%d", &n) != 1) {
		printf("not number\n");
		return 1;
	}
	if (n < 0) {
		printf("nonnegative only\n");
		return 1;
	}
	countdown(n);

	return 0;
}

/* print numbers from n down through 0 (does nothing if n < 0) */
void countdown(int n) {
	if (n >= 0) {
		printf("%d\n", n);
		if (n > 0) countdown(n-1);
	}
}