#include <bits/stdc++.h>
using namespace std;
void solve() {
  string a, b, c;
  long long cnt = 0;
  cin >> a >> b;
  for (long long i = 0; i < (long long)a.size(); i++) {
    if (a[i] == b[i]) {
      c += a[i];
    } else {
      cnt++;
      (cnt % 2) ? c += a[i] : c += b[i];
    }
  }
  (cnt % 2) ? cout << "impossible" : cout << c;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  for (long long i = 0; i < 1; i++) {
    solve();
    cout << '\n';
  }
  return 0;
}
