#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 233;
int k;
char s[maxn], t[maxn];
int a[maxn];
int main() {
  ios_base::sync_with_stdio(false);
  cin >> k;
  cin >> s + 1;
  cin >> t + 1;
  for (int i = 1; i <= k; i++) {
    s[i] -= 'a';
    t[i] -= 'a';
  }
  for (int i = k; i >= 1; i--) {
    a[i] += t[i] + s[i];
    if (a[i] >= 26) a[i] -= 26, a[i - 1]++;
  }
  for (int i = 0; i <= k; i++) {
    if (a[i] & 1) a[i + 1] += 26;
    a[i] /= 2;
  }
  for (int i = 1; i <= k; i++) {
    s[i] = a[i] + 'a';
  }
  cout << s + 1 << endl;
  return 0;
}
