class Hamming {
  int distance(String a, String b) {
    if (a.length != b.length)
      throw ArgumentError('strands must be of equal length');
    if (a == b) return 0;
    int difference = 0;
    for (int i = 0; i < a.length; i++) {
      if (a[i] != b[i]) difference++;
    }
    return difference;
  }
}