#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int i, j, t, ans = 0;
  string s, s1;
  cin >> s;
  cin >> s1;
  int l = s.length();
  int l1 = s1.length();
  int count = 0;
  for (i = 0; i < l; i++) {
    if (s[i] != s1[i]) count++;
  }
  if (count % 2 != 0)
    cout << "impossible" << endl;
  else {
    for (i = 0; i < l; i++) {
      if (s[i] != s1[i]) {
        ans++;
        if (ans % 2 != 0) {
          if (s[i] == '0')
            s[i] = '1';
          else if (s[i] == '1')
            s[i] = '0';
        }
      }
    }
    cout << s << endl;
  }
}
