#include <bits/stdc++.h>
using namespace std;
int v[100005];
int main() {
  string s, s2, s3 = "";
  cin >> s >> s2;
  int n = s.size(), cnt = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] != s2[i]) cnt++;
  }
  if (cnt % 2)
    cout << "impossible" << endl;
  else {
    int cnt2 = n - cnt, cnt3 = cnt / 2, cnt4 = cnt3;
    for (int i = 0; i < n; i++) {
      if (s[i] == s2[i])
        s3 += s[i];
      else
        s3 += '0';
    }
    for (int i = 0; cnt3 && i < n; i++) {
      if (s[i] != s2[i]) {
        v[i] = true;
        cnt3--;
        s3[i] = s[i];
      }
    }
    for (int i = 0; cnt4 && i < n; i++) {
      if (!v[i] && s[i] != s2[i]) {
        v[i] = true;
        cnt4--;
        s3[i] = s2[i];
      }
    }
    for (int i = 0; cnt3 && cnt4 && i < n; i++) {
      if (s[i] == s2[i]) {
        if (s[i] == '1')
          s3[i] = '0';
        else if (s[i] == '0')
          s3[i] == '1';
        cnt3--;
        cnt4--;
      }
    }
    cout << s3 << endl;
  }
  return 0;
}
