replacing a missing value in R with average value

I have a dataframe with columns of data with missing value and I would like to replace the missing value by taking the mean using the value of the cells above and below.

 df1<-c(2,2,NA,10, 20, NA,3)
 if(df1[i]== NA){
 df1[i]= mean(df1[i+1],df1[i-1])
}

However, I am getting this error

  Error in if (df1[i] == NA) { : missing value where TRUE/FALSE needed
  In addition: Warning message:
  In if (df1[i] == NA) { :
  the condition has length > 1 and only the first element will be used

Any guidance would be appreciated to solve this issue.