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.
# Enter your code here. Read input from STDIN. Print output to STDOUT
import math

if __name__ == '__main__':
    a = input().strip()
    b = input().strip()
    c = []
    swap = False
    sc = 0
    for x, y in zip(a, b):
    	if x == y:
    		c.append(x)
    	else:
    		if swap:
    			c.append(y)
    		else:
    			c.append(x)
    		swap = not swap
    		sc += 1
    if sc % 2 == 0:
    	print("".join(c))
    else:
    	print("impossible")
  	 	   			  		  		 	  	 		  		