PROJECT EULER #77

Link to Project Euler problem 77

It is possible to write ten as the sum of primes in exactly five different ways:

7 + 3
5 + 5
5 + 3 + 2
3 + 3 + 2 + 2
2 + 2 + 2 + 2 + 2

What is the first value which can be written as the sum of primes in over five thousand different ways?

A modification of the previous algorithm

using System;
using System.Collections.Generic;

namespace ProjectEuler
{
    class Program
    {
        static void Main()
        {
            //Problem 77
            DateTime start = DateTime.Now;
            var primes = GeneratePrimes(80);
            int maxNumber = 80,target=0,sum=0;
            int[,] a = new int[maxNumber + 1, maxNumber + 1];
            for (int i = 0; i < maxNumber + 1; i++)
            {
                a[i, 0] = 0;
                a[0, i] = 1;
            }
            int n = 2;
            while (sum < 5000)
            {
                    if (n%2 == 0) a[n, 1] = 1;
                    else a[n, 1] = 0;
                    for (int j = 2; j < primes.Count; j++)
                    {
                        int k = n - primes[j];
                        if (k < 0)
                            a[n, j] = a[n, j - 1];
                        else
                            a[n, j] = a[n, j - 1] + a[k, j];
                        if (a[n, j] > 5000)
                        {
                            target = n;
                            sum = a[n, j];
                            break;
                        }
                    }
                n++;
            }
            Console.WriteLine(sum + "  " +target);
            TimeSpan time = DateTime.Now - start;
            Console.WriteLine("This took {0}", time);
            Console.ReadKey();
        }
        public static bool IsPrime(int n)
        {
            if (n < 2) return false;
            if (n == 2) return true;
            for (long i = 2; i <= Math.Sqrt(n); i++)
                if (n % i == 0) return false;
            return true;
        }
        public static List<int> GeneratePrimes(int n)
        {
            var primeNumbers = new List<int> { 0,2, 3 };
            for (int i = 5; i <= n; i += 2)
                if (IsPrime(i))
                    primeNumbers.Add(i);
            return primeNumbers;
        }
    }
}