#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  string s1, s2;
  cin >> s1 >> s2;
  valarray<int> a;
  a.resize(n, 0);
  for (int i = 0; i < n; ++i) a[i] = s1[n - i - 1] - 'a' + s2[n - i - 1] - 'a';
  for (int i = 0; i < n - 1; ++i) {
    a[i + 1] += a[i] / 26;
    a[i] %= 26;
  }
  int dlt = 0;
  for (int i = n - 1; i >= 0; --i) {
    dlt *= 26;
    int cur = a[i] + dlt;
    cout << static_cast<char>((cur >> 1) + 'a');
    dlt = cur & 1;
  }
  return 0;
}
