#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s, t;
  cin >> s >> t;
  int x = s.size();
  vector<int> count_s;
  for (int i = 0; i < x; i++) {
    count_s.push_back(s[i] - 'a');
  }
  vector<int> count_t;
  for (int i = 0; i < x; i++) {
    count_t.push_back(t[i] - 'a');
  }
  for (int i = n - 1; i >= 0; i--) {
    count_s[i] += count_t[i];
    if (i) {
      count_s[i - 1] += count_s[i] / 26;
      count_s[i] = count_s[i] % 26;
    }
  }
  for (int i = 0; i < n; i++) {
    int rem = count_s[i] % 2;
    count_s[i] = count_s[i] / 2;
    if (i + 1 < n) count_s[i + 1] += rem * 26;
  }
  for (int i = 0; i < n; i++) cout << char('a' + count_s[i]);
  cout << endl;
  return 0;
}
