k.columns=['Sid','Tstamp','Itemid','Category']
k=k.loc[:,('Sid','Tstamp')]
idx=k.groupby(['Sid'])['Tstamp'].transform(max) == k['Tstamp']
ah=k[idx].reset_index()
idy=k.groupby(['Sid'])['Tstamp'].transform(min) == k['Tstamp']
ai=k[idy].reset_index()
kgrp=k.groupby('Sid').count()

i=0
for temp1,temp2 in zip(ah['Tstamp'],ai['Tstamp']):
    sv1= datetime.datetime.strptime(temp1, "%Y-%m-%dT%H:%M:%S.%fZ")
    sv2= datetime.datetime.strptime(temp2, "%Y-%m-%dT%H:%M:%S.%fZ")
    d1=time.mktime(sv1.timetuple()) + (sv1.microsecond / 1000000.0)
    d2=time.mktime(sv2.timetuple()) + (sv2.microsecond / 1000000.0)
    kgrp.loc[i,'duration']= d1-d2
    i=i+1