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.
str_1, str_2 = input(), input()
new_string = ""
listOf_indices = 0
a = True
for i in range(len(str_1)):
    if str_1[i] != str_2[i]:
        if a:
            new_string += str_1[i]
        else:
            new_string += str_2[i]
        a = not a
        listOf_indices += 1
    else:
        new_string += str_1[i]
if listOf_indices%2 != 0:
    print("impossible")
else:
    print(new_string)
        
