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.
a = input()
b = input() 
x = str(int(a) ^ int(b))
if x.count("1") %2 != 0 :
    print("IMPOSSIBLE")
else : 
    res = ""
    flag1 = True  
    for idx , value  in enumerate(x) : 
        if value == "0":
            res += str(a)[idx] 
        else: 
            if flag1 :
                res += "1"
                flag1 =False
            else:
                res += "0"
                flag1 = True
    print(res)    