#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  string p(s.size(), '0');
  int dif = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != t[i]) dif++;
  }
  if (dif % 2 != 0) return cout << "impossible", 0;
  int a(0), b(0);
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != t[i]) {
      if (a <= b) {
        p[i] = t[i];
        a++;
      } else {
        p[i] = s[i];
        b++;
      }
    }
  }
  cout << p;
}
