gnome terminal – How can I tell the command I want the “nest” value

I want to know how can I tell the Linux terminal that I want to work with the “next” value in the “set”. For example:

for i in `seq 1 2 20`; do     echo $i; done

results in:

1
3
5
7
9
11
13
15
17
19

What if I want the result to be:

3
5
7
9
11
13
15
17
19
"Error"

Assuming the last one would be an error because there is no other value after 19.

In advance, thanks for the help.