#include <bits/stdc++.h>
using namespace std;
vector<int> ans;
int main() {
  int t = 0;
  string s[2], ans;
  cin >> s[0] >> s[1];
  for (int i = 0; i < s[0].size(); i++) {
    if (s[0][i] != s[1][i]) {
      ans += s[t][i];
      t = (t + 1) % 2;
    } else {
      ans += s[0][i];
    }
  }
  if (t == 1)
    cout << "impossible";
  else {
    for (int i = 0; i < ans.size(); i++) cout << ans[i];
  }
  return 0;
}
