How to find any non positive integer

[noob]

So I need to return the value '-1' when s = any non-digits (other than spaces).
my code so far looks like :

def parseVote(s=None):
    if not s:
         print('0')  
    elif s >= 0:
         print(s)
    else :
        print(-1)

But its the middle 'elif' that needs changing to include letters. 
Thankyou!!

---

Actually might as well post whole question to make it easier.
def parseVote(s):
parseVote(s) returns the vote from s. Return 0 for an empty vote, and -1 if there are any non-digits (other than spaces). For example,
parseVote("") = parseVote(" ") = 0,
parseVote("-3") = parseVote("no") = parseVote("1 5") = -1,
parseVote("15") = parseVote(" 15 ") = 15.