#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  string s, t;
  cin >> n >> s >> t;
  vector<int> a(n + 1);
  for (int i = 0; i < n; ++i)
    a[i] = (s[n - i - 1] - 'a') + (t[n - i - 1] - 'a');
  for (int i = 0; i < n; ++i) {
    if (a[i] >= 26) {
      ++a[i + 1];
      a[i] -= 26;
    }
  }
  vector<int> b(n + 1);
  b[0] = a[0] / 2;
  for (int i = 1; i <= n; ++i) {
    b[i] = a[i] / 2;
    if (a[i] & 1) b[i - 1] += 13;
  }
  for (int i = n - 1; i >= 0; --i) cout << char('a' + b[i]);
}
