#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e18;
const long long mod = 1e9 + 7;
inline long long bigmod(long long B, long long P) {
  long long R = 1;
  while (P > 0) {
    if (P & 1) {
      R = (R * B) % mod;
    }
    P >>= 1;
    B = (B * B) % mod;
  }
  return R;
}
const long long mxn = 2e5 + 10;
long long Mod(long long a) { return a % mod; }
long long Mul(long long a, long long b) { return Mod(Mod(a) * Mod(b)); }
long long Add(long long a, long long b) { return Mod(Mod(a) + Mod(b)); }
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  long long tt = 1;
  for (long long tc = 1; tc <= tt; tc++) {
    string s, t, p = "";
    cin >> s >> t;
    long long n = s.size();
    long long sf = 1, c = 0;
    for (long long i = 0; i < n; i++) {
      if (s[i] == t[i])
        p += s[i];
      else if (sf) {
        p += t[i];
        sf = 0;
        c++;
      } else {
        p += s[i];
        sf = 1;
        c++;
      }
    }
    if (c % 2)
      cout << "impossible\n";
    else
      cout << p << '\n';
  }
  return 0;
}
