#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 10;
char s1[maxn], s2[maxn], ans[maxn];
int main() {
  while (~scanf("%s", s1)) {
    scanf("%s", s2);
    int len = strlen(s1), cnt = 0;
    for (int i = 0; i < len; i++) {
      if (s1[i] != s2[i]) {
        cnt++;
        if (cnt & 1)
          ans[i] = s1[i];
        else
          ans[i] = s2[i];
      } else
        ans[i] = s1[i];
    }
    ans[len] = 0;
    if (cnt % 2 == 1) {
      printf("impossible\n");
      continue;
    }
    printf("%s\n", ans);
  }
  return 0;
}
