Please Help me to do this Sum and split array like this.

int[] arr = { 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 4 };
Console.WriteLine(arr.Length);
int split = 2;
List<int[]> splitted = new List<int[]>();
for (int i = 0; i < split; i++)
{
splitted.Add(new int[arr.Length/2]);
for (int j = i*arr.Length/2; j < (i * arr.Length / 2) +(arr.Length/2); j++)
{
(splitted[i])[j-(i * arr.Length / 2)] = arr[j];
}
Console.WriteLine();
}
List<int> sums = new List<int>();
foreach (int[] item in splitted)
{
for(int k=0;k<3;k++)
{
int sum = 0;
for (int i = k; i < item.Length; i+=3)
{
sum += item[i];
}
sums.Add(sum);
}
}
foreach (int sum in sums)
{
Console.WriteLine(sum);
}