Solution for https://app.codesignal.com/challenge/dW3QTjvRxcJaGnNXE

Solution.cs

int sequenceElement(int[] a, int n)
{
    int number = a[0] * 10000 + a[1] * 1000 + a[2] * 100 + a[3] * 10 + a[4];
    int beginer = number;

    if (n < 5)
    {
        return a[n];
    }
    var list = new List<int>();
    do
    {
        list.Add(number);

        number = (number % 10000) * 10 
                + ((number / 10000) % 10
                + (number / 1000) % 10
                + (number / 100) % 10
                + (number / 10) % 10
                + (number / 1) % 10) % 10;

    } while (beginer != number);

    return list[(n - 4) % list.Count] % 10;
}