#include <bits/stdc++.h>
using namespace std;
int main() {
  string str1;
  cin >> str1;
  string str2;
  cin >> str2;
  string output = "";
  int count = 0;
  for (int i = 0; i < str1.size(); i++) {
    if (str1[i] == str2[i]) {
      output += str1[i];
    } else {
      count++;
      if (count % 2 == 1) {
        output += str1[i];
      } else {
        output += str2[i];
      }
    }
  }
  if (count % 2 == 1) {
    cout << "impossible" << endl;
  } else {
    cout << output << endl;
  }
  return 0;
}
