#include <bits/stdc++.h>
using namespace std;
struct money {
  int d;
  int c;
};
string a, b;
int main() {
  cin >> a >> b;
  int bvt = 0;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] != b[i]) {
      bvt++;
    }
  }
  if (bvt % 2 == 1) {
    cout << "impossible" << endl;
    return 0;
  }
  int cot = 0;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] == b[i]) {
      cout << a[i];
    }
    if (a[i] != b[i]) {
      if (cot % 2 == 0) {
        cout << a[i];
        cot++;
      } else {
        cout << b[i];
        cot++;
      }
    }
  }
  return 0;
}
