using System;
using System.Linq;
namespace Fibonacci
{
    class Program
    {
        
        static void Main(string[] args)
        {
            
            string number;
            bool check = true;
            Console.Write("Enter fibonacci sequence: ");
            do
            {
                number = Console.ReadLine();
                if (checkInputData(number))
                {
                    Console.WriteLine("invalid number");
                    Console.Write("Enter again: ");
                }
                else
                    check = false;

            } while (check);

            
            if (checkFibonacci(number))
                Console.WriteLine("string is fibonacci sequence");
            else
                Console.WriteLine("string is not fibonacci sequence");

        }

        // ======check input data=====
        // don't accept non-numeric characters
        // fibonacci must be start 0 1 or 1 1
        static bool checkInputData(string number)
        {
            bool check = false;
            string[] convert = number.Split(" ");
             if(convert[0] == "0" && convert[1] != "1" || convert[0] == "1" && convert[1] != "1" || convert.Length < 3)
               {
                 check = true;
               }
          else
            {
              for (int i = 0, intType; i < convert.Length; i++)
           	  {
                if (Int32.TryParse(convert[i], out intType) == false)
                {                
                    check = true;
                    break;
                }
              }
            }
            

            return check;
        }

        //====check Fibonacci========
        static bool checkFibonacci(string number)
        {
            
            bool result = true;
            int sum;
            int[] fibonacci = number.Split().Select(int.Parse).ToArray();
            for (int i = 0; i < fibonacci.Length - 2; i++)
            {
                sum = fibonacci[i] + fibonacci[i + 1];  

                if (fibonacci[i + 2] != sum)
                {
                    result = false;
                    break;
                }              
            }

            return result;
        }
    }
}