#include <bits/stdc++.h>
using namespace std;
void solve(string &s1, string &s2, int n) {
  vector<int> res(n + 1, 0);
  for (int i = n; i > 0; i--) {
    res[i] += s1[i - 1] - 'a' + s2[i - 1] - 'a';
    if (i) {
      res[i - 1] += res[i] / 26;
      res[i] %= 26;
    }
  }
  int p = 0;
  for (int i = 0; i <= n; i++) {
    p = p * 26 + res[i];
    res[i] = p / 2;
    p = p % 2;
  }
  for (int i = 1; i <= n; i++) {
    cout << char(res[i] + 'a');
  }
}
int main() {
  int n;
  cin >> n;
  string s1, s2;
  cin >> s1 >> s2;
  solve(s1, s2, n);
  return 0;
}
