#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  int diffCnt = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != t[i]) diffCnt++;
  }
  if (diffCnt % 2 == 1) {
    cout << "impossible" << endl;
  } else {
    string ans = "";
    for (int i = 0; i < s.size(); i++) {
      ans += "@";
    }
    for (int i = 0; i < s.size(); i++) {
      if (s[i] == t[i]) ans[i] = s[i];
    }
    int cur = 0;
    for (int i = 0; i < s.size(); i++) {
      if (cur == 0 && ans[i] == '@') {
        ans[i] = s[i];
        cur = 1;
      } else if (cur == 1 && ans[i] == '@') {
        ans[i] = t[i];
        cur = 0;
      }
    }
    cout << ans << endl;
  }
  return 0;
}
