#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  {
    string s, t, a, b;
    cin >> s >> t;
    int cnt = 0;
    bool u = 0, cm;
    for (long long i = 0, j = 0; i < s.size(); i++) {
      if (s[i] - t[i] && j) {
        if (s[i] < t[i]) {
          if (cm) {
            cm = 0;
            b.push_back(b[j - 1]);
            a.push_back(b[j - 1]);
          } else {
            b.push_back(!(b[j - 1] - '0') + '0');
            a.push_back(!(b[j - 1] - '0') + '0');
          }
        } else if (s[i] > t[i]) {
          if (!cm) {
            cm = 1;
            b.push_back(b[j - 1]);
            a.push_back(b[j - 1]);
          } else {
            b.push_back(!(b[j - 1] - '0') + '0');
            a.push_back(!(b[j - 1] - '0') + '0');
          }
        }
        j++;
        cnt++;
      } else if (s[i] - t[i]) {
        cnt++;
        j++;
        b.push_back('0');
        a.push_back('0');
        if (s[i] < t[i]) {
          cm = 0;
        } else
          cm = 1;
      } else {
        a.push_back(s[i]);
      }
    }
    if (cnt % 2) {
      cout << "impossible\n";
    } else {
      cout << a << endl;
    }
  }
}
