Is there a better way to identify a single missing value in a vector? Why does the L disappear when I add 1?

Question

I made a script that identifies a single missing value in a vector.

c<-c(1:489,491:1000)
n<-c(1:length(c))
missing<-length(which(c==n))+1

When I do not add the one, the output is

489L

I do not know what the L represents.

When I add the one in the length argument, it gets ignored

> c<-c(1:489,491:1000)
> n<-c(1:length(c))
> missing<-length(which(c==n)+1)
> missing
[1] 489

But the object in my global environment is

489L

So what is the meaning of the L? Why is the 1 not recognised in the final instance? Is there a better way to perform this operation?