#include <bits/stdc++.h>
using namespace std;
long long n, m;
const long long N = 200011;
long long a[N];
string s1;
string s2;
vector<long long> v1;
vector<long long> v2;
signed main() {
  ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> n;
  cin >> s1 >> s2;
  for (long long i = n - 1; i >= 0; i--) {
    v1.push_back(s1[i] - ('a'));
    v2.push_back(s2[i] - ('a'));
  }
  long long d = 0;
  vector<long long> res;
  for (long long i = 0; i < v1.size(); i++) {
    long long sum = v1[i] + v2[i] + d;
    res.push_back(sum % 26);
    d = sum / 26;
  }
  if (d != 0) {
    res.push_back(d);
  }
  vector<long long> ans;
  long long x = 0;
  for (long long i = res.size() - 1; i >= 0; i--) {
    x += res[i];
    ans.push_back(x / 2);
    x %= 2;
    x *= 26;
  }
  vector<char> gud;
  for (long long i = 0; i < ans.size(); i++) {
    gud.push_back(char(ans[i] + ('a')));
  }
  long long col = n - gud.size();
  for (long long i = 1; i <= col; i++) {
    cout << 'a';
  }
  bool fl = 1;
  for (long long i = 0; i < gud.size(); i++) {
    if (gud[i] != 'a') {
      fl = 0;
    }
    if (gud[i] != 'a') {
      cout << gud[i];
    } else {
      if (!fl || gud.size() == n) {
        cout << gud[i];
      }
    }
  }
  return 0;
}
