number validation in python?

So working on a small program that will randomly give me numbers depending on what number I put into the terminal just I want it to only allow numbers, someone gave me this:

from random import randrange
s = input('Enter a Number: ')
if s.isdigit(): n = int( s )
else: n = -1
print(randrange(n + 1))

but it just gives me a traceback error still saying:

raise ValueError("empty range for randrange()") ValueError: empty range for randrange()

and ive also tried this:

from random import randrange
s = input('Enter a Number: ')
if s.isdigit():
    print('Enter a number instead of a letter') 
n = int(  )
print(randrange(n + 1))

and its then printing this out without an error so im stuffed

**Enter a Number: s

0

OK BYE NOW**

any help?