#include <bits/stdc++.h>
using namespace std;
string get_median(int n, string s, string t);
int main() {
  int N;
  cin >> N;
  string s;
  cin >> s;
  string t;
  cin >> t;
  string p = get_median(N, s, t);
  cout << p << endl;
}
void p(vector<int> a) {
  for (auto d : a) {
    cout << d << ",";
  }
  cout << endl;
}
string get_median(int n, string s, string t) {
  vector<int> si;
  long int s_number = 0;
  for (int i = 0; i < n; i++) {
    int k = s[i] - 'a';
    si.push_back(k);
  }
  long int t_number = 0;
  vector<int> ti;
  for (int i = 0; i < n; i++) {
    int k = t[i] - 'a';
    ti.push_back(k);
  }
  vector<int> ai;
  int carry = 0;
  for (int i = 0; i < n; i++) {
    int x = (ti[i] + si[i]) / 2 + carry;
    ai.push_back(x);
    if ((ti[i] + si[i]) % 2 == 1) {
      carry = 13;
    } else {
      carry = 0;
    }
  }
  string ans;
  for (int i = n - 1; i >= 0; i--) {
    int x = ai[i];
    if (x >= 26) {
      x -= 26;
      ai[i - 1] += 1;
    }
    ans += x + 'a';
  }
  reverse(ans.begin(), ans.end());
  return ans;
}
