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

void countup(int start, int end);

int main(void) {

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

	return 0;
}

/* print numbers from 0 up through n (does nothing if n < 0) */
void countup(int start, int end) {
	if (start > end) { }
	else {
		printf("%d\n", start);
		countup(start+1, end);
	}
}