#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t, p;
  bool flag = false;
  int dist = 0;
  cin >> s >> t;
  int len = strlen(s.c_str());
  for (int i = (0); i < (len); ++i) {
    if (s[i] != t[i]) {
      ++dist;
      if (flag)
        p.push_back(s[i]), flag = false;
      else
        p.push_back(t[i]), flag = true;
    } else {
      p.push_back(s[i]);
    }
  }
  if (dist % 2)
    cout << "impossible" << endl;
  else
    cout << p << endl;
  return 0;
}
