There is a Pandas dataframe: userId movieId rating timestamp 0 1 31 2.5 1260759144 1 1 1029 3.0 1260759179 2 1 1061 3.0 1260759182 3 1 1129 2.0 1260759185 4 1 1172 4.0 1260759205 Calculate the average lifetime of users. The lifetime is the difference between the maximum and minimum values of the "timestamp" column for a given "userId".

df=pd.DataFrame({'userId':[1,1,1,1,1],
                 'movieId':[31,1029,1061,1129,1172],
                 'rating':[2.5,3,3,2,4],
                 'timestamp':[1260759144,1260759179,1260759182,1260759185,1260759205]
                })
df