Project Euler #6 has been answered more times than problems 3-5, which is generally interpreted as meaning that it's easier than they. It reads:

The sum of the squares of the first ten natural numbers is,

12 + 22 + ... + 102 = 385

The square of the sum of the first ten natural numbers is,

(1 + 2 + ... + 10)2 = 552 = 3025

Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 - 385 = 2640.

Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.

100 is definitely low enough to allow a brute-force solution, so that's how I approached the problem. Two variables churned through a single, 100-iteration for loop.

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

namespace Problem6
{
    class Program
    {
        static void Main(string[] args)
        {
            int sumOfSquares = 0;
            long squareOfSums = 0;

            for (int i = 1; i <= 100; i++)
            {
                sumOfSquares += (i * i);
                squareOfSums += i;
            }

            squareOfSums *= squareOfSums;

            Console.WriteLine("Difference: {0}",(squareOfSums - sumOfSquares));
        }
    }
}