#include <bits/stdc++.h>
using namespace std;
int len;
char A[(100000 + 10)], B[(100000 + 10)], ans[(100000 + 10)];
int use[(100000 + 10)], G = 0, g = 0;
int main() {
  scanf("%s", A);
  scanf("%s", B);
  len = strlen(A);
  for (int i = 0; i < len; ++i)
    if (A[i] != B[i]) {
      use[i] = 1;
      ++G;
    };
  if (G % 2) {
    printf("impossible\n");
    exit(0);
  };
  for (int i = 0; i < len; ++i)
    if (use[i]) {
      if ((++g) % 2)
        printf("%c", A[i]);
      else
        printf("%c", B[i]);
    } else
      printf("%c", A[i]);
  return 0;
}
