Sheet1 - logged ticket books to sales persons

name beginno endno
a 51 100
b 101 150

I do below checks by vba if user makes a wrong entry say

d 75 - msgbox " Number already in use "
e 151 141 msgbox " Wrong entry"

But couldn't do below check

f 1 200 msgbox " Numbers already in
use"

Any help ? Thank you