#include <bits/stdc++.h>
using PII = std::pair<int, int>;
using LL = long long;
using VI = std::vector<int>;
using CVI = const VI;
using VLL = std::vector<LL>;
using VVI = std::vector<VI>;
using VVLL = std::vector<VLL>;
using namespace std;
int main() {
  int n;
  string s, t;
  cin >> n >> s >> t;
  reverse((s).begin(), (s).end());
  reverse((t).begin(), (t).end());
  for (int i = 0; i < s.size(); ++i) {
    s[i] = s[i] - 'a' + t[i] - 'a';
    if (s[i] > 25) {
      s[i] -= 26;
      if (i + 1 == s.size())
        s.push_back(1);
      else
        ++s[i + 1];
    }
  }
  reverse((s).begin(), (s).end());
  int c = 0;
  for (auto i = 0; i < ((int)((s).size())); ++i) {
    if (c) {
      s[i] += 26;
      c = 0;
    }
    c = s[i] & 1;
    s[i] = (s[i] >> 1) + 'a';
  }
  cout << s.substr((int)((s).size()) - n) << endl;
  return 0;
}
