```python
def root_digit(value: int) -> int:
    """
    Fold a whole count down to a single figure by adding its figures together,
    then adding the figures of that answer, again and again until one figure is left.
    A count already standing as one figure is given back as it is.
    
    :param value: The integer to fold.
    :return: The final single figure.
    """
    if value < 10:
        return value
    total = 0
    while value > 0:
        total += value % 10
        value = total
    return total
```