abstract class AbstractStringBuilder {
    char value[];
    int count;
	
    public AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

    public AbstractStringBuilder append(CharSequence s, int start, int end) {
        if (s == null)
            s = "null";
        if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        if (len == 0)
            return this;
        int newCount = count + len;
        if (newCount > value.length)
            expandCapacity(newCount);
        for (int i=start; i<end; i++)
            value[count++] = s.charAt(i);
        count = newCount;
        return this;
    }

    void expandCapacity(int minimumCapacity) {
        int newCapacity = (value.length + 1) * 2;
        if (newCapacity < 0) {
            newCapacity = Integer.MAX_VALUE;
        } else if (minimumCapacity > newCapacity) {
            newCapacity = minimumCapacity;
        }
        value = Arrays.copyOf(value, newCapacity);
    }
    
    public int capacity() {
        return value.length;
    }
    
    public abstract String toString();
}

class StringBuilder extends AbstractStringBuilder {
    public StringBuilder(int capacity) {
        super(capacity);
    }
    
    public String toString() {
        return new String(value, 0, count);
    }
}