#include <stdio.h>
#include <conio.h>


int is_prime(int n) {
    // We know that 2 and 3 are primes. So if we get
    // 2 or 3 then directly return 1 (true)
    if((n == 2) || (n == 3))
        return 1;

    for(int i=2; i<=n/2; ++i) {
        // If i divides n any time, then the number is not prime, return 0 (false)
        if(n%i == 0)
            reutrn 0;
    }

    // If we reach here that means the number is either prime or 1.
    // Since 1 is not prime, check for it and return accordingly
    return n!=1;
}


void main() {
    clrscr();
    for(int i=1; i<=20; ++i) {
        if(is_prime(i))
            printf("%d\n", i);
    }
    getch();
}