#include <bits/stdc++.h>
using namespace std;
int main() {
  size_t n;
  string s, t;
  cin >> n >> s >> t;
  int base = ('z' - 'a') + 1;
  vector<int> l(n + 1), r(n + 1);
  for (int i = 0; i < int(n); ++i) {
    l[i + 1] = s[i] - 'a';
  }
  for (int i = 0; i < int(n); ++i) {
    r[i + 1] = t[i] - 'a';
  }
  for (int i = n; i >= 0; --i) {
    l[i] += r[i];
    if (i > 0) {
      l[i - 1] += l[i] / 26;
      l[i] %= 26;
    }
  }
  vector<int> res(n + 1);
  for (int i = 0; i <= n; ++i) {
    int rem = l[i] % 2;
    l[i] /= 2;
    if (i < n) {
      l[i + 1] += rem * 26;
    }
  }
  for (int i = 1; i <= n; ++i) {
    cout << char(l[i] + 'a');
  }
  cout << endl;
  return 0;
}
