#include <bits/stdc++.h>
using namespace std;
void jabru() {
  string a, b;
  cin >> a >> b;
  int n = a.size();
  string ans = "";
  int ind = 0;
  for (long long i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      ans += a[i];
    } else {
      if (ind % 2)
        ans += a[i];
      else
        ans += b[i];
      ind++;
    }
  }
  if (ind % 2)
    cout << "impossible"
         << "\n";
  else
    cout << ans << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  jabru();
  return 0;
}
