#include <bits/stdc++.h>
using namespace std;
string in[2], p;
int main() {
  cin >> in[0] >> in[1];
  bool change = 1;
  int dis = 0;
  for (int i = 0; i < in[0].length(); i++) dis += in[0][i] != in[1][i];
  if (dis % 2) {
    cout << "impossible";
    return 0;
  }
  for (int i = 0; i < in[0].length(); i++) {
    if (in[0][i] == in[1][i])
      p += in[0][i];
    else
      p += in[change = !change][i];
  }
  cout << p;
  return 0;
}
