#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &r) {
  os << "(" << r.first << "," << r.second << ")";
  return os;
}
inline int nxt() {
  int first;
  scanf("%d", &first);
  return first;
}
long long gcd(long long a, long long b) {
  while (b > 0) {
    a %= b;
    swap(a, b);
  }
  return a;
}
template <typename T>
T sqr(T a) {
  return a * a;
}
const long long mod = 1000000009;
long long pw(long long a, long long n) {
  long long ret = 1;
  while (n) {
    if (n & 1) ret = ret * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return ret;
}
long long inv(long long a) { return pw(a, mod - 2); }
void solve() {
  string a, b;
  cin >> a >> b;
  string ans = a;
  int n = a.length();
  int cnt = 0;
  for (int i = 0; i < n; ++i) {
    if (a[i] != b[i]) {
      if (cnt & 1) {
        ans[i] = b[i];
      }
      ++cnt;
    }
  }
  if (cnt & 1) {
    cout << "impossible\n";
  } else {
    cout << ans << "\n";
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
