Remove characters occurring multiple times

A string S is passed as the input. The program must remove all characters which appear more than once. If all the characters in a string are occurring more than once, then print -1 as output. 

Input Format: The first line contains the value of S. 

Boundary Conditions: 2 <= Length of S <= 100 

Output Format: The string with the characters occurring more than once removed. 

Example Input/Output 1: 

Input: 

level

Output: 

v 

Explanation: l and e occur more than once. Hence they are removed. 

Example Input/Output 2:
Input: 

mistake 

Output: 

mistake 

Example Input/Output 3: 

Input: 

ababab 

Output:

-1

Source Code:

s=input()

l=[a for a in s if s.count(a)==1]

l=''.join(str(i) for i in l)

if len(l)==0:

    print("-1")

else:

    print(l)