task 5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter a whole number");
            uint n = uint.Parse(Console.ReadLine());

            int first = 0;
            int second = 1;
            int third = 0;
            int sum = 0;

            for (int i = 0; i < n - 1; i++)
            {
                first = second;
                second = third;
                third = first + second;
                sum = sum + third;
            }
            Console.WriteLine("\nsum of first {0} number in Fibonacci's sequence = {1}\n\n", n, sum);
        }
    }
}