[R] return NA

[dscully@fd9ns01.okladot.state.ok.us]

A<-c(1,2,NA,7,5)
B<-c(3,4,1,4,1)
C<-c(6,5,6,NA,9)
D<-c(8,7,4,6,2)
df1<-cbind(A,B,C,D)
for(i in seq(1,ncol(df1)-1, by=2)) {
   ifelse(df1[,i]=="NA",df1[,i+1]=="NA",df1[,] )  }


Tried several variations but none worked.  I wish to find any NA's in
column's 1 or 3 and change the numerical value to the right  of  the "
NA"'s .  In this case I wish to replace the 1 and 6 respectively with "NA".
Any help would be appreciated.