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.


# def GetHammingDistance(firstString: str, secondString: str, length: int) -> int:
    
#     result = 0
    
#     for index in range(0, length):
#         if firstString[index] != secondString[index]:
#             result += 1

#     return result

def GetInfo(firstString: str, secondString: str, length : int) -> list:
    
    indexs = []
    
    for index in range(0, length):
        if firstString[index] != secondString[index]:
            indexs.append(index)
    
    if firstString.count("1") > secondString.count("1"):
        indexs.append(firstString)
    
    else:
        indexs.append(firstString)
        
    return indexs



def Solution():

    firstString = input()
    secondString = input()

    stringsLength = len(firstString)

    if stringsLength % 2 != 0:
        return "impossible"
    
    else:
        data = GetInfo(firstString, secondString, stringsLength)
        
        dataLength = len(data)
        
        if (dataLength - 1) % 2 != 0:
            return "impossible"
        
        smallString = data[len(data) - 1]
        
        resultString = ""
        
        for element in range(0, dataLength - 2):
            index = data[element]
            
            resultString = smallString[:index] + "1" + smallString[index + 1:]
            
        return resultString
    


        
        
print(Solution())
        
    
    



    
    