too slow problem working with array

double c;
       double e;
       double E;
       double o;
       double v;


       public void compute(int[,] arr)
       {
           c = 0;
           e = 0;
           E = 0;
           o = 0;
           v = 0;

           for (int i = 0; i < 256; i++)
               for (int j = 0; j < 256; j++)
               {
                   {

                       c += Math.Pow(i - j, 2) * arr[i, j];
                       e += Math.Pow(arr[i, j], 2);
                       if (arr[i, j] != 0)
                       {
                           E += arr[i, j] * Math.Log(arr[i, j], 2);
                       }
                       o += (arr[i, j] / (1 + Math.Abs(i - j)));
                       v += ((Math.Pow((arr[i, j] - get_p(arr)), 2)) / 256);

                   }
               }

       }
       public double get_p(int[,] arr)
       {
           double p = 0;

           for (int i = 0; i < 256; i++)
               for (int j = 0; j < 256; j++)
               {
                   {


                       p += arr[i, j] / 256;

                   }
               }

           return p;
       }