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.
def solve(s1,s2):

    n = len(s1)
    c = 0

    for i in range(n):
        if s1[i]!=s2[i]:
            c+=1

    if c%2==1:
        return('impossible')
    
    else:
        r = ""
        idx = 0
        while c>0:
            if s1[idx]=='0':
                r+='1'
            else:
                r+='0'

            c -= 2

    r = r + s1[len(r):]
    return r



s1 = input()
s2 = input()

print(solve(s1,s2))
   	   	 	  			 			     			 		