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.
s = input()
t = input()
flag = 1
cnt = 0
for i in range(len(s)):
    if s[i] != t[i]:
        cnt += 1
if cnt % 2 == 1:
    print("impossible")
else:
    flag = 0
    c = ""
    for i in range(len(s)):
        if s[i] == t[i]:
            c = c + s[i]
        else:
            if flag:
                c = c + s[i]
                flag = 0
            else:
                c = c + t[i]
                flag = 1
    print(c)    