#include <bits/stdc++.h>
long long int MAX(long long int a, long long int b) {
  if (a > b)
    return a;
  else
    return b;
}
using namespace std;
int main() {
  string S, SS;
  cin >> S >> SS;
  int PR = 0, DIF = 0, l = S.length();
  for (int x = 0; x < l; x++)
    if (S[x] != SS[x]) DIF++;
  if (DIF & 1)
    printf("impossible\n");
  else {
    DIF /= 2;
    bool SEC = false;
    for (int i = 0; i < l; i++) {
      if (S[i] == SS[i])
        printf("1");
      else {
        if (PR == DIF) SEC = true;
        if (!SEC) {
          if (S[i] == '1')
            printf("0");
          else
            printf("1");
        } else {
          if (SS[i] == '1')
            printf("0");
          else
            printf("1");
        }
        PR++;
      }
    }
    printf("\n");
  }
  return 0;
}
