class Tape {
  char[] right;
  char[] left;
  Tape() {
    right = new char[0];
    left = new char[0];
  }
  char get(int index) {
    if (index >= 0) {
      if (index >= right.length) {
        return BLANK;
      }
      return right[index];
    }
    else
    {
      if (-index - 1 >= left.length) {
        return BLANK;
      }
      return left[-index - 1];
    }
  }
  void set(int index, char state) {
    if (index >= 0) {
      if (index >= right.length) {
        right = expand(right, index + 1);
      }
      right[index] = state;
    }
    else
    {
      if (-index - 1 >= left.length) {
        left = expand(left, -index);
      }
      left[-index - 1] = state;
    }
  }
}

static final char BLANK = ' ';