def strlen(s):
    """
    Returns the length of string s using recursion.

    Examples:
        >>> strlen("input")
        5
        >>> strlen("")
        0
        >>> strlen('123456789')
        9
        >>> strlen("I love software engineering")
        27
        >>> strlen('a'*527)
        527
    """
    if s == "":
        return 0
    else:
        return strlen(s[1:]) + 1