#include <bits/stdc++.h>
using namespace std;
long long T, n, gg[200100], a[200100], b[200100], ans[200100];
string str;
int main() {
  ios::sync_with_stdio(0);
  long long i, j;
  cin >> n;
  cin >> str;
  for (i = 1; i <= n; i++) a[i] = str[i - 1] - 'a';
  cin >> str;
  for (i = 1; i <= n; i++) b[i] = str[i - 1] - 'a';
  for (i = 1; i <= n; i++) {
    ans[i] += (a[i] + b[i]) / 2;
    if ((a[i] + b[i]) % 2) {
      ans[i + 1] += 13;
    }
  }
  for (i = n; i >= 1; i--) {
    ans[i - 1] += ans[i] / 26;
    ans[i] %= 26;
  }
  for (i = 1; i <= n; i++) {
    printf("%c", (char)(ans[i] + 'a'));
  }
}
