Combing two pandas dataframes

Question

I have two pandas dataframes I would like to combine as follows:

input:

df1:

x	y
x1	y1
x2	y2
x3	y3
x4	y4
x5	y5
x6	y6

df2:

x	y	z
x2	y2	z2
x3	y3	z3
x4	y4	z4

Expected output:

x	y	z
x1	y1	Nan
x2	y2	z2
x3	y3	z3
x4	y4	z4
x5	y5	Nan
x6	y6	Nan

Any suggestions?