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.
s1 = input()
s2 = input()
nd = 0
for i in range(len(s1)):
	if s1[i] != s2[i]:
		nd+=1
if nd%2 != 0:
	print('impossible')
else:
	sm = []
	n2 = 0
	for i in range(len(s1)):
		if s1[i] != s2[i]:
			if n2 < nd/2:
				sm.append(s1[i])
				n2 = n2+1
			else:
				sm.append(s2[i])
		else:
			sm.append(s1[i])
	print(''.join(sm))
  	  				    	 		  			 		 	 			