#include <bits/stdc++.h>
using namespace std;
int main() {
  string s[2];
  cin >> s[0] >> s[1];
  string ans(s[0]);
  int parity = 0, n = s[0].size();
  for (int i = 0; i < n; i++) {
    if (s[0][i] != s[1][i]) {
      ans[i] = s[parity][i];
      parity ^= 1;
    }
  }
  if (parity == 0) {
    cout << ans << endl;
  } else {
    cout << "impossible" << endl;
  }
}
