```python
def line_number(block: str) -> str:
    lines = block.splitlines()
    if not lines:
        return ''
    
    max_length = max(len(line) for line in lines)
    result = ''
    
    for i line in lines:
        line_number = str(len(line) + 1).
        result += f"{line_number}:{line}\n'
    
    return result
```