Problem: Little Susie loves strings. Today she calculates distances between them. As Susie is a small girl after all, her strings contain only digits zero and one. She uses the definition of Hamming distance:

We will define the distance between two strings s and t of the same length consisting of digits zero and one as the number of positions i, such that si isn't equal to ti. 

As besides everything else Susie loves symmetry, she wants to find for two strings s and t of length n such string p of length n, that the distance from p to s was equal to the distance from p to t.

It's time for Susie to go to bed, help her find such string p or state that it is impossible.

Input

The first line contains string s of length n. 

The second line contains string t of length n.

The length of string n is within range from 1 to 105. It is guaranteed that both strings contain only digits zero and one.

Output

Print a string of length n, consisting of digits zero and one, that meets the problem statement. If no such string exist, print on a single line "impossible" (without the quotes).

If there are multiple possible answers, print any of them.

Examples

Input

0001
1011


Output

0011


Input

000
111


Output

impossible

Note

In the first sample different answers are possible, namely — 0010, 0011, 0110, 0111, 1000, 1001, 1100, 1101.

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
long long int ceil1(long long int n, long long int m) {
  if (n % m == 0) {
    return n / m;
  }
  return n / m + 1;
}
vector<long long int> factor(long long int n) {
  vector<long long int> ans;
  if (n > 0) ans.push_back(1);
  if (n == 2) ans.push_back(2);
  for (long long int i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ans.push_back(i);
      if (n / i != i) ans.push_back(n / i);
    }
  }
  if (n > 2) ans.push_back(n);
  return ans;
}
long long int fact(long long int n) {
  long long int res = 1;
  for (long long int i = 2; i <= n; i++) res = res * i;
  return res;
}
int nCr(long long int n, long long int r) {
  return fact(n) / (fact(r) * fact(n - r));
}
void dfs(vector<vector<long long int>> &v, long long int k,
         vector<long long int> &par, vector<long long int> &dis,
         long long int p) {
  par[k] = p;
  dis[k] = dis[p] + 1;
  for (long long int i = 0; i < v[k].size(); i++) {
    if (v[k][i] != p) {
      dfs(v, v[k][i], par, dis, k);
    }
  }
}
bool sortbysec(const pair<string, int> &a, const pair<string, int> &b) {
  return (a.second < b.second);
}
long long int ston(string s) {
  stringstream geek(s);
  long long int x = 0;
  geek >> x;
  return x;
}
long long int pow2(long long int n, long long int k) {
  long long int ans = 1;
  while (k > 0) {
    if (k % 2 == 1) ans = ans * n;
    n = n * n;
    k /= 2;
  }
  return ans;
}
bool prime(int n) {
  int i, j;
  for (i = 2; i <= sqrt(n); i++) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}
bool bserch(vector<string> a, string s) {
  long long int l = 0, r = a.size() - 1, mid;
  while (l <= r) {
    mid = (l + r) / 2;
    if (a[mid] == s) return true;
    if (a[mid] < s) {
      l = mid + 1;
    } else {
      r = mid - 1;
    }
  }
  return false;
}
bool sortcol(const vector<int> &v1, const vector<int> &v2) {
  return v1[0] < v2[0];
}
long long int degit(long long int n) {
  long long int t = 0;
  while (n > 0) {
    t++;
    n /= 10;
  }
  return t;
}
void solve() {
  string s, t;
  string p = "";
  cin >> s >> t;
  long long int dif = 0;
  for (long long int i = 0; i < s.size(); i++)
    if (s[i] != t[i]) dif++;
  if (dif % 2 == 1)
    cout << "impossible";
  else {
    long long int fl = 0;
    for (long long int i = 0; i < s.size(); i++) {
      if (s[i] != t[i]) {
        if (fl == 0)
          p += s[i], fl = 1;
        else
          p += t[i], fl = 0;
      } else
        p += s[i];
    }
    cout << p;
  }
  return;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int q;
  q = 1;
  while (q--) {
    solve();
  }
  return 0;
}