/// <summary>
        /// Searches a string for all characters that are in <see cref="Array" />, and returns the number of occurrences of all characters.
        /// </summary>
        /// <param name="str">String to search.</param>
        /// <param name="chars">One-dimensional, zero-based <see cref="Array"/> that contains characters to search for.</param>
        /// <returns>The number of occurrences of all characters.</returns>
        public static int GetCharsCount(string str, char[] chars)
        {
            // #1. Implement the method using "for" statement.
            if (chars is null)
            {
                throw new ArgumentNullException(nameof(chars));
            }

            if (str is null)
            {
                throw new ArgumentNullException(nameof(str));
            }

            int counter = 0;

            for (int i = 0;  i < chars.Length; i++)
            {
                for (int k = 0; k < str.Length; k++)
                {
                    if (chars[i] == str[k])
                    {
                        counter++;
                    }
                }
            }

            return counter;
        }