from gobble import *

@parser
def natural():
    digits = yield character('0123456789').star
    return int(''.join(digits))

@parser
def literal_null():
    yield literal('NULL')
    return None

literal = natural / literal_null

@parser
def natural():
    sign = yield character('-+').option('+')
    factor = {'-': -1, '+': 1}[sign]
    value = yield natural
    return value * factor

whitespace = character(' \n\r\t').star

literal_expr = literal << whitespace

value = literal_expr.execute(input_string)
print(value)