#include <bits/stdc++.h>
using namespace std;
int n, add;
vector<int> a, b, res;
char s[200005], t[200005];
int main() {
  cin >> n;
  scanf("%s", s);
  scanf("%s", t);
  for (int i = n - 1; i >= 0; i--) {
    b.push_back(s[i] - 'a');
    a.push_back(t[i] - 'a');
  }
  a.push_back(0);
  b.push_back(0);
  a.push_back(0);
  b.push_back(0);
  for (int i = 0; i < a.size() - 1; i++) {
    a[i + 1] += (a[i] + b[i]) / 26;
    a[i] = (a[i] + b[i]) % 26;
  }
  reverse(a.begin(), a.end());
  for (int i = 0; i < a.size(); i++) {
    res.push_back((add * 26 + a[i]) / 2);
    add = (add * 26 + a[i]) % 2;
  }
  reverse(res.begin(), res.end());
  while (res.size() > n) res.pop_back();
  reverse(res.begin(), res.end());
  for (int i = 0; i < res.size(); i++) cout << (char)(res[i] + 'a');
  cout << endl;
  return 0;
}
