#include <bits/stdc++.h>
using namespace std;
int code(char c) { return c - '0'; }
void solve() {
  long long a, b;
  string s1, s2;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < s1.size(); ++i) {
    if (code(s1[i]) ^ code(s2[i])) {
      cnt++;
    }
  }
  if (cnt & 1) {
    cout << "impossible" << endl;
  } else {
    cnt >>= 1;
    for (int i = 0; i < s1.size() && cnt > 0; ++i) {
      if (code(s1[i]) ^ code(s2[i])) {
        s1[i] = s2[i];
        cnt--;
      }
    }
    cout << s1 << endl;
  }
}
int main() {
  cout.setf(ios::fixed);
  cout.precision(9);
  solve();
  return 0;
}
