#include <bits/stdc++.h>
char s[2][100010], t[100010];
int main() {
  int i, j, k, len;
  scanf("%s%s", s[0], s[1]);
  len = strlen(s[0]);
  k = 0;
  for (i = 0; i < len; i++) {
    if (s[0][i] == s[1][i]) {
      t[i] = s[0][i];
    } else {
      t[i] = s[k][i];
      k = (k + 1) % 2;
    }
  }
  if (k % 2 == 0) {
    printf("%s\n", t);
  } else {
    printf("impossible\n");
  }
  return 0;
}
