#include <iostream>
using namespace std;

int main()
{
 cout << "Please input an even number in range [4,30]: ";

 int number;
 cin >> number;

 while ( number < 4  ||  number > 30  ||  number % 2 != 0 )
 {
   cout << "Invalid.  Please try again: ";
   cin >> number;
 }

 int spaces = 20;

 for ( int c = 2 ; c <= number ; c += 2 )
 {
   // display the initial spaces:
   for ( int s = 0 ; s < spaces ; s++ ) {
     cout << ' ';
   }

   // display the asterisk characters:
   for ( int i = 0 ; i < c ; i++ ) {
     cout << '*';
   }
   cout << endl;

   spaces--;
 }

 return 0;
}