from typing import List
import hashlib


def md5_hashsum(file_names: List[str]) -> str:
    """
    Calculate md5 hash sum of files listed

    Args:
        file_names: list of file names

    Returns:
        hashsum string
    """
    hash_md5 = hashlib.md5()
    for file_name in file_names:
        with open(file_name, "rb") as f:
            for chunk in iter(lambda: f.read(4096), b""):
                hash_md5.update(chunk)
    return hash_md5.hexdigest()