#include <bits/stdc++.h>
using namespace std;
string itosm(long long x) {
  if (x == 0) return "0";
  string ans = "";
  while (x > 0) {
    ans += ((x % 10) + '0');
    x /= 10;
  }
  reverse(ans.begin(), ans.end());
  return ans;
}
long long stoim(string str) {
  long long ans = 0;
  long long k = 1;
  for (int i = str.length() - 1; i >= 0; i--) {
    ans += (str[i] - '0') * k;
    k *= 10;
  }
  return ans;
}
const long long infll = 1e18 + 3;
const int inf = 1009000999;
const double eps = 1e-6;
const int maxn = 1e6 + 77;
const int baseint = 1000200013;
const long long basell = 1e18 + 3;
const long double PI = acos(-1.0);
int main() {
  srand(228228);
  ios_base::sync_with_stdio(0);
  ;
  string t, s, p = "";
  cin >> t >> s;
  int n = s.length();
  while (p.length() != n) p += "0";
  int cnt = 0;
  for (int i = (0); i < (n); i++) {
    if (s[i] == t[i])
      p[i] = s[i];
    else
      cnt++;
  }
  if (cnt % 2) {
    cout << "impossible";
    return 0;
  }
  cnt = 0;
  for (int i = (0); i < (n); i++) {
    if (s[i] != t[i]) {
      if (cnt <= 0) {
        p[i] = s[i];
        cnt++;
      } else {
        p[i] = t[i];
        cnt--;
      }
    }
  }
  cout << p;
  return 0;
}
