Little Susie loves strings. Today she calculates distances between them. As Susie is a small girl after all, her strings contain only digits zero and one. She uses the definition of Hamming distance:

We will define the distance between two strings *s* and *t* of the same length consisting of digits zero and one as the number of positions *i*, such that *s*<sub class="lower-index">*i*</sub> isn't equal to *t*<sub class="lower-index">*i*</sub>. 

As besides everything else Susie loves symmetry, she wants to find for two strings *s* and *t* of length *n* such string *p* of length *n*, that the distance from *p* to *s* was equal to the distance from *p* to *t*.

It's time for Susie to go to bed, help her find such string *p* or state that it is impossible.
The first line contains string *s* of length *n*. 

The second line contains string *t* of length *n*.

The length of string *n* is within range from 1 to 10<sup class="upper-index">5</sup>. It is guaranteed that both strings contain only digits zero and one.
from sys import stdin ,stdout 
input=stdin.readline
def print(*args, end='\n', sep=' ') -> None:
    stdout.write(sep.join(map(str, args)) + end)

s1=input().strip() ; s2=input().strip() ; ans="" ; res=0
for i , j in zip(s1,s2):
    if i==j:
        ans+=i
    else:
        if res%2==0:
            ans+=i
        else:
            ans+=j
        res+=1
if res%2==0:
    print(ans)
else:
    print("impossible")