def check_gap(xpub, api_key):
params = {'<STR_LIT:key>': api_key, '<STR_LIT>': xpub}<EOL>resource = '<STR_LIT>' + util.urlencode(params)<EOL>resp = util.call_api(resource, base_url='<STR_LIT>')<EOL>json_resp = json.loads(resp)<EOL>return json_resp['<STR_LIT>']<EOL>
Call the 'v2/receive/checkgap' endpoint and returns the callback log
    for a given callback URI with parameters.

    :param str xpub: extended public key
    :param str api_key: Blockchain.info API V2 key
    :return: an int