#include <bits/stdc++.h>   using namespace std;   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     string str1, str2;     cin >> str1 >> str2;     int diffcount = 0;     for (int i = 0; i < str1.length(); i++) {       if (str1[i] != str2[i]) diffcount++;     }     if (diffcount % 2 == 1) {       cout <<  impossible ;       return 0;     }     int index = 0;     int toreach = diffcount / 2;     while (diffcount != toreach) {       if (str1[index] != str2[index]) {         if (str1[index] ==  1 )           str1[index] =  0 ;         else           str1[index] =  1 ;         diffcount--;       }       index++;     }     cout << str1;   }   