Project Euler Problem 28

Problem definition-
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Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:
21 22 23 24 25
20 7 8 9 10
19 6 1 2 11
18 5 4 3 12
17 16 15 14 13
It can be verified that the sum of both diagonals is 101.
What is the sum of both diagonals in a 1001 by 1001 spiral formed in the same way?
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C# brute-force solution:

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static long SpiralDiagonalsSum(long spiralsize)
{
  long sum = 1;
  long cursize = 3;
  long number = 1;
  while (cursize <= spiralsize)
  {
    for (int i = 0; i < 4; i++)
    {
      number += cursize - 1;
      sum += number;
    }
    cursize += 2;
  }
  return sum;
}
static void Main(string[]args)
{
  System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
  sw.Start();
  long diagsum = SpiralDiagonalsSum(1001);
  sw.Stop();
  Console.WriteLine("Spiral diagonals sum {0} calculated in {1} ms", diagsum, sw.Elapsed.TotalMilliseconds);
  Console.ReadLine();
}

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Solution on core duo performs in a 0.2 ms.