#include <bits/stdc++.h>
using namespace std;
int k;
char s[200005], t[200005];
int ans[200005];
int main() {
  cin >> k >> s >> t;
  for (int i = k - 1; i > 0; i--) {
    ans[i] += s[i] - 'a' + t[i] - 'a';
    ans[i - 1] += ans[i] / 26;
    ans[i] %= 26;
  }
  ans[0] += s[0] - 'a' + t[0] - 'a';
  for (int i = 0; i < k; i++) {
    if (ans[i] % 2 == 1) ans[i + 1] += 26;
    ans[i] /= 2;
  }
  for (int i = 0; i < k; i++) printf("%c", ans[i] + 'a');
  cout << endl;
  return 0;
}
