[R] not common records

[Ashta]

I have 2 data sets.  File1 and File2. Some records are common to both
data sets. For those common records I want get the difference between
d_x1z1= z1-x1   and d_x2z2= z2-x2.

File1<- data.frame(var = c(561,752,800,900),  x1= c(23,35,40,15), x2=
c(125,284,280,347))
File2<- data.frame(var = c(561,752,800,1001), z1= c(43,45,40,65), z2=
c(185,299,280,310))

Record  900      15    347   appears only in File1
Record   1001    65    310  appears only in File2

File3 should look like as follows

File3
var      x1     x2     z1    z2    d_x1z1   d_x2z2
561      23    125  43    165     20         40
752      35    284  45    299      8          15
800      40    280  40    280      0           0
900      15    347  NA   NA      NA       NA
1001     NA  NA   65    310     NA      NA

How do I get those record not common in both data sets ?
merge( File1,File2) gave me only for common "var"