#include <bits/stdc++.h>
using namespace std;
int fastread() {
  int t = 0;
  char ch = getchar();
  while (ch < 33) ch = getchar();
  while (ch > 33) {
    t = t * 10 + ch - '0';
    ch = getchar();
  }
  return t;
}
int main() {
  int i = 0, j, k, n, sum;
  char a[100000], b[100000];
  char ch = getchar();
  while (ch < 33) ch = getchar();
  while (ch > 33) {
    a[i++] = ch;
    ch = getchar();
  }
  n = i;
  for (i = 0; i < n; i++) b[i] = getchar();
  getchar();
  sum = 0;
  for (i = 0; i < n; i++)
    if (a[i] != b[i]) sum++;
  if (sum % 2)
    printf("impossible\n");
  else {
    for (i = 0, k = 0; i < n && k < sum / 2; i++) {
      if (a[i] != b[i]) {
        if (a[i] == '0')
          a[i] = '1';
        else
          a[i] = '0';
        k++;
      }
    }
    for (i = 0; i < n; i++) printf("%c", a[i]);
    printf("\n");
  }
}
