#include <stdio.h>

int main() {
   int n;
    printf("enter the value of n: \n");
    if (scanf("%d", &n) != 1) {
        printf("invalid.");
    } else if (n <= 0) {
        printf("error.");
    } else {
        printf("%d is a natural number \n", n);
        }
    

    return 0;
}