#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, p;
  long long i, j, k, cnt, sum;
  cin >> a >> b;
  cnt = 0;
  for (i = 0; i < a.size(); i++) {
    if (a[i] != b[i]) cnt++;
  }
  if (cnt % 2) {
    printf("impossible");
    return 0;
  }
  int flag = 0;
  for (i = 0; i < a.size(); i++) {
    if (a[i] == b[i])
      p.push_back(a[i]);
    else {
      if (!flag)
        p.push_back(a[i]);
      else
        p.push_back(b[i]);
      flag = 1 - flag;
    }
  }
  cout << p;
  return 0;
}
