#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  string s3;
  int count = 0;
  bool first0_second1 = false;
  for (int i = 0; i < s1.length(); i++) {
    if (s1[i] != s2[i]) {
      count++;
      if (first0_second1) {
        s3.insert(s3.end(), s2[i]);
        first0_second1 = false;
      } else {
        s3.insert(s3.end(), s1[i]);
        first0_second1 = true;
      }
    } else {
      s3.insert(s3.end(), s1[i]);
    }
  }
  if (count % 2 == 0)
    cout << s3 << endl;
  else
    cout << "impossible" << endl;
  return 0;
}
