#include <bits/stdc++.h>
using namespace std;
long long n, i, j, k, l, m, arr[200010], x, a, b, c, d, arr1[200010], p, hi, lo,
    mid, s, y, t;
map<long long, long long> mp, mp1;
vector<long long> vc, vc1;
string st, st1, st2;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> st >> st1;
  n = st.size();
  m = 0;
  l = 0;
  for (i = 0; i < n; i++) {
    if (st[i] != st1[i]) {
      m++;
      if (m % 2 == 0)
        st2 += st[i];
      else
        st2 += st1[i];
      l++;
    } else
      st2 += st[i];
  }
  if (l % 2 == 0)
    cout << st2;
  else
    cout << "impossible";
}
