#include <bits/stdc++.h>
using namespace std;
int n;
int a[200005], b[200005];
int s[200005] = {0};
char c;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> c;
    a[n - i + 1] = c - 'a';
  }
  for (int i = 1; i <= n; i++) {
    cin >> c;
    b[n - i + 1] = c - 'a';
  }
  for (int i = 1; i <= n; i++) {
    s[i] += a[i] + b[i];
    s[i + 1] += s[i] / 26;
    s[i] %= 26;
  }
  for (int i = n + 1; i >= 1; i--) {
    s[i - 1] += (s[i] & 1) * 26;
    s[i] = s[i] / 2;
  }
  for (int i = n; i >= 1; i--) printf("%c", s[i] + 'a');
  return 0;
}
