#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < s1.size(); i++) {
    if (s1[i] != s2[i]) {
      cnt++;
    }
  }
  if (cnt % 2 == 1) {
    cout << "impossible";
  } else {
    int cnt1 = 0, cnt2 = 0, cnt = 0;
    int o = 0, z = 0;
    string s3 = "";
    for (int i = 0; i < s1.size(); i++) {
      if (s1[i] == s2[i]) {
        s3 = s3 + "0";
      } else {
        if (cnt % 2 == 0) {
          s3 = s3 + s1[i];
        } else {
          s3 = s3 + s2[i];
        }
        cnt++;
      }
    }
    cout << s3;
  }
  cout << endl;
}
