#include <bits/stdc++.h>
using namespace std;
long long a, b, n;
string s, t, e;
bool key = true;
char c, c2, c3;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> s;
  cin >> t;
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != t[i]) a++;
  }
  if (a % 2 == 1) {
    cout << "impossible";
    return 0;
  }
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != t[i] && a > 0) {
      if (key) {
        e = e + s[i];
        key = false;
        continue;
      } else {
        e = e + t[i];
        key = true;
        continue;
      }
    } else {
      e = e + s[i];
    }
  }
  cout << e;
  return 0;
}
