static long MaxDoubleSliceSum(int[] a)
{
    var size = a.Length;
    if (size < 3)
    {
        return 0;
    }

    var p1 = new long[size];
    var p2 = new long[size];
    p1[1] = 0;
    p2[size - 2] = 0;

    for (int i = 2; i < size - 1; i++)
    {
        p1[i] = Math.Max(0, p1[i - 1] + a[i - 1]);
        p2[size - i - 1] = Math.Max(0, p2[size - i] + a[size - i]);
    }

    var sum = p1[1] + p2[1];
    for (int i = 1; i < size - 1; i++)
    {
        sum = Math.Max(sum, p1[i] + p2[i]);
    }

    return sum;
}