#include <bits/stdc++.h>
using namespace std;
int num[200002];
void norm(int k) {
  for (int i = 0; i < k; i++) {
    while (num[i] > ('z' - 'a')) {
      num[i + 1]++;
      num[i] -= ('z' - 'a' + 1);
    }
  }
}
void dumpnum(int k) {
  for (int i = 0; i <= k; i++) cout << num[i] << " ";
  cout << endl;
}
int main() {
  int k;
  cin >> k;
  string s1, s2;
  string ans;
  cin >> s1;
  cin >> s2;
  for (int i = 0; i < k; i++) {
    num[i] = s1[k - i - 1] - 'a' + s2[k - i - 1] - 'a';
  }
  norm(k);
  for (int i = k; i >= 0; i--) {
    if (i > 0 && num[i] % 2 == 1) num[i - 1] += ('z' - 'a' + 1);
    num[i] /= 2;
  }
  norm(k);
  for (int i = k - 1; i >= 0; i--) ans += (char)('a' + num[i]);
  cout << ans << endl;
}
