#include <bits/stdc++.h>
using namespace std;
char a[100005], c[100005], k[100005];
int main() {
  int len, i, j, cnt;
  while (cin >> a >> c) {
    len = strlen(a);
    cnt = 0;
    for (i = 0; i < len; i++) {
      if (a[i] != c[i]) cnt++;
    }
    if (cnt & 1)
      cout << "impossible" << endl;
    else {
      cnt /= 2;
      for (i = 0; i < len && cnt; i++) {
        if (a[i] != c[i]) cnt--;
        k[i] = a[i];
      }
      while (i < len) {
        if ('0' == a[i])
          k[i] = '1';
        else
          k[i] = '0';
        i++;
      }
      k[i] = '\0';
      cout << k << endl;
    }
  }
  return 0;
}
