многопоток

using System;
using System.Diagnostics;
using System.Threading;

namespace ConsoleApp1
{
    class ConsoleApp1
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Введите кол-во строк: ");
            int n = Convert.ToInt32(Console.ReadLine());
            Console.WriteLine("Введите кол-во столбцов: ");
            int m = Convert.ToInt32(Console.ReadLine());
            double N = 0;
            double t1 = 0, t2 = 0, t3 = 0, t4 = 0;
            double s=0;
            var timer = Stopwatch.StartNew();
            void FirstThread()
            {
                for (int i = 1; i < (((n ) / 4) + 1); i++)
                {
                    s = 0;
                    for (int j = 1; j < m + 1; j++)
                    {
                        s = Math.Sqrt((i-1)+(j-1));
                        t1 += s;
                    }
                }
            }
            void SecondThread()
            {
                for (int i = (((n) / 4) + 1); i < ((n) / 2) + 1; i++)
                {
                    s = 0;
                    for (int j = 1; j < m + 1; j++)
                    {
                        s = Math.Sqrt((i-1)+(j-1));
                        t2 += s;
                    }
                }
            }
            void ThirdThread()
            {
                for (int i = ((n) / 2) + 1; i < ((3 * (n)) / 4) + 1; i++)
                {
                    s = 0;
                    for (int j = 1; j < m + 1; j++)
                    {
                        s = Math.Sqrt((i-1)+(j-1));
                        t3 += s;
                    }
                }
            }
            void FourthThread()
            {
                for (int i = (3 * (n)) / 4 + 1; i < (n + 1); i++)
                {
                    s = 0;
                    for (int j = 1; j < m + 1; j++)
                    {
                        s = Math.Sqrt((i-1)+(j-1));
                        t4 += s;
                    }
                }
            }
            Thread thr1 = new Thread(() => FirstThread());
            Thread thr2 = new Thread(() => SecondThread());
            Thread thr3 = new Thread(() => ThirdThread());
            Thread thr4 = new Thread(() => FourthThread());
            thr1.Start();
            Thread.Sleep(100);
            thr2.Start();
            Thread.Sleep(100);
            thr3.Start();
            Thread.Sleep(100);
            thr4.Start();
            Thread.Sleep(100);
            N = t1 * t2 * t3 * t4;
            Console.WriteLine("Результат - " + (N - N % 0.001));
            timer.Stop();
            Console.WriteLine("Выполнение метода заняло мс: {0}", timer.ElapsedMilliseconds - 400);
            Console.WriteLine("Выполнение метода заняло тактов: {0}", timer.ElapsedTicks-400000);
            Console.ReadKey();
        }
    }
}