#include <bits/stdc++.h>
using namespace std;
long long pw(long long b, long long p) {
  if (!p) return 1;
  long long sq = pw(b, p / 2);
  sq *= sq;
  if (p % 2) sq *= b;
  return sq;
}
long long gcd(long long a, long long b) { return (b == 0 ? a : gcd(b, a % b)); }
long long sd(long long x) { return x < 10 ? x : x % 10 + sd(x / 10); }
long long lcm(long long a, long long b) { return ((a * b) / gcd(a, b)); }
int main() {
  string s, t, p;
  cin >> s >> t;
  int cnt = 0, n = ((int)((s).size()));
  for (int i = 0; i < n; ++i) {
    if (s[i] != t[i]) {
      ++cnt;
      if (cnt % 2)
        p.push_back(s[i]);
      else
        p.push_back(t[i]);
    } else
      p.push_back(s[i]);
  }
  if (cnt % 2 == 0)
    cout << p;
  else
    cout << "impossible";
}
