if (chars.Length > 1)
{
char ch = chars[1];

if (ch >= 0xAC00 && ch <= 0xD7A3)
{
  int code = ch - 0xAC00;

  int cho = code / (21 * 28);
  int jung = (code % (21 * 28)) / 28;
  int jong = code % 28;

  if (jong == 4)
  {
    jong = 21;
    chars[1] = (char)(0xAC00 + (cho * 21 * 28) + (jung * 28) + jong);
  }
}