#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 50;
int ans[maxn];
int main() {
  std::ios::sync_with_stdio(false);
  int n;
  cin >> n;
  char c;
  for (int i = 1; i <= n; i++) {
    cin >> c;
    ans[i] += c - 'a';
  }
  for (int i = 1; i <= n; i++) {
    cin >> c;
    ans[i] += c - 'a';
  }
  for (int i = n; i >= 1; i--) {
    if (ans[i] >= 26) {
      ans[i - 1] += ans[i] / 26;
      ans[i] %= 26;
    }
  }
  for (int i = 0; i <= n; i++) {
    ans[i + 1] += ans[i] % 2 * 26;
    ans[i] /= 2;
  }
  for (int i = 1; i <= n; i++) {
    cout << (char)(ans[i] + 'a');
  }
  cout << endl;
  return 0;
}
