How to get max value for a specific key from a python dictionary

I have a python dict as below:

a={  u'data': [    [u'2013-03-19', u'1363762799', u'4004658'],    [u'2013-03-20', u'1363849199', u'4756668']  ],  u'columns': [    u'dt_pst',    u'timestamp',    u'count'  ]}

How can I get the max value for key u'timestamp'? I want this number: 1363849199.

Thanks!