#include <bits/stdc++.h>
using namespace std;
template <class T1, class T2>
ostream& operator<<(ostream& out, pair<T1, T2>& p) {
  out << p.first << ' ' << p.second;
}
template <class T>
istream& operator>>(istream& in, vector<T>& v) {
  for (auto& x : v) in >> x;
  return in;
}
template <class T>
ostream& operator<<(ostream& out, vector<T>& v) {
  for (auto x : v) out << x << ' ';
  return out;
}
int main() {
  iostream::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long int n;
  cin >> n;
  string s, t;
  cin >> s >> t;
  vector<long long int> v1, v2;
  for (long long i = (0); i < (n); ++i) {
    v1.push_back(s[i] - 'a');
    v2.push_back(t[i] - 'a');
  }
  vector<long long int> v;
  long long int c = 0;
  for (long long int i = n - 1; i >= 0; i--) {
    v.push_back((c + v1[i] + v2[i]) % 26);
    if (c + v1[i] + v2[i] >= 26)
      c = (c + v1[i] + v2[i]) / 26;
    else
      c = 0;
  }
  if (c > 0) v.push_back(c);
  reverse((v).begin(), (v).end());
  cerr << "v"
       << ": " << v << "\n";
  ;
  vector<long long int> vv;
  long long int nm = 0;
  for (long long int i = 0; i < v.size(); i++) {
    nm = nm * 26 + v[i];
    if (nm == 1 && i != 0) {
      vv.push_back(0);
    } else if (nm == 1)
      continue;
    else {
      vv.push_back(nm / 2);
      nm = nm % 2;
    }
  }
  for (long long i = (0); i < (vv.size()); ++i) {
    cout << char('a' + vv[i]);
  }
}
