#include <bits/stdc++.h>
using namespace std;
vector<int> get(const string &s) {
  vector<int> res(s.size() + 1);
  for (int i = 0; i < int(s.size()); ++i) {
    res[i + 1] = s[i] - 'a';
  }
  return res;
}
int main() {
  int k;
  string s, t;
  cin >> k >> s >> t;
  vector<int> a = get(s), b = get(t);
  for (int i = k; i >= 0; --i) {
    a[i] += b[i];
    if (i) {
      a[i - 1] += a[i] / 26;
      a[i] %= 26;
    }
  }
  for (int i = 0; i <= k; ++i) {
    int rem = a[i] % 2;
    a[i] /= 2;
    if (i + 1 <= k) {
      a[i + 1] += rem * 26;
    } else {
      assert(rem == 0);
    }
  }
  for (int i = 1; i <= k; ++i) {
    cout << char('a' + a[i]);
  }
  cout << endl;
  return 0;
}
