KDoubleSubstrings.cs

    using System.Text;

    public class KDoubleSubstrings
    {
        public int howMuch(string[] str, int k)
        {
            int result = 0;
            StringBuilder stringBuilder = new StringBuilder();

            foreach (string s in str)
            {
                stringBuilder.Append(s);
            }

            string all = stringBuilder.ToString();

            for (int step = 1; step <= all.Length / 2; step++)
            {
                for (int index = 0; index <= all.Length - (2 * step); index++)
                {
                    int currentK = CalculateK(GetSubstring(index, step, all));

                    if (currentK <= k)
                    {
                        result++;
                    }
                }
            }

            return result;
        }

        public string GetSubstring(int index, int step, string all)
        {
            string firstHalf = all.Substring(index, step);
            string secondHalf = all.Substring(index + step, step);

            return firstHalf + secondHalf;
        }

        public int CalculateK(string str)
        {
            int lenght = (str.Length / 2);
            string firstHalf = str.Substring(0, lenght);
            string secondHalf = str.Substring(lenght, lenght);

            int result = 0;

            for (int i = 0; i < lenght; i++)
            {
                if (firstHalf[i] != secondHalf[i])
                {
                    result++;
                }
            }

            return result;
        }
    }