#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  cin >> a >> b;
  int d = 0;
  string ans = "";
  int prev = 0, cnt = 0;
  for (int i = 0; i < a.length(); ++i) {
    if (a[i] == b[i]) {
      ans += a[i];
    } else {
      cnt++;
      if (prev == 0) {
        ans += a[i];
        prev = 1;
      } else {
        ans += b[i];
        prev = 0;
      }
    }
  }
  if (cnt & 1)
    cout << "impossible";
  else
    cout << ans;
  return 0;
}
