botanics

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Numerics;

namespace Q10223
{
    class Program
    {
        static private int n; static private bool ntof; static private BigInteger[] bi;
        static void Main(string[] args)
        {
            for (; ; )
            {
            labeln: Console.Write("n>>>");
                ntof = int.TryParse(Console.ReadLine(), out n);
                if (!ntof || n <= 0) goto labeln;
                if (n == 1) { Console.WriteLine("Ans=1"); goto labeln; }
                bi = new BigInteger[n + 1]; bi[0] = 1;
                for (int i = 1; bi[i - 1] <= n; i++)
                {
                    bi[i] = bi[i - 1] * (4 * i - 2) / (i + 1); 
                    if(bi[i]==n)Console.WriteLine("Ans={0}",i);
                }
            }
        }
    }
}