#include <bits/stdc++.h>
using namespace std;
const int N = 102;
string ss, tt, pp;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie();
  cout.tie();
  cin >> ss >> tt;
  int ii = 0;
  for (int i = 0; i < ss.size(); i++) {
    if (ss[i] == tt[i])
      pp += ss[i];
    else {
      ++ii;
      if (ii % 2 == 1) {
        int x = (ss[i] - '0') - (tt[i] - '0');
        if (x < 0) {
          pp += '0';
        } else
          pp += '1';
      } else {
        int x = (tt[i] - '0') - (ss[i] - '0');
        if (x < 0) {
          pp += '0';
        } else
          pp += '1';
      }
    }
  }
  if (ii % 2 == 0)
    cout << pp;
  else
    cout << "impossible";
}
