[ Telerik C#] LOOPS - EX 12

// 12. Write a program that reads from the console a positive integer
// number N (N < 20) and outputs a matrix.
using System;

namespace ex12
{
    class Program
    {
        static void Main(string[] args)
        {
            int n;
            do
            {
                Console.Write("Enter N: ");
                n = Int32.Parse(Console.ReadLine());
            } while (!(n < 20 && n > 0));

            for (int i = 0; i < n; i++)
            {
                for (int j = 1 + i; j <= n+i; j++)
                {
                    Console.Write(j);
                }
                Console.WriteLine();
            }
        }
    }
}