#include <bits/stdc++.h>
using namespace std;
map<char, int> mp;
int main() {
  int n;
  cin >> n;
  string s1, s2, s3;
  cin >> s1 >> s2;
  for (int i = 0; i < 26; i++) {
    char m = i + 97;
    mp[m] = i;
  }
  int flag = 0, ans = 0;
  for (int i = n - 1; i >= 0; i--) {
    ans++;
    s3 += ((mp[s1[i]] + mp[s2[i]] + flag) % 26 / 2 + 97);
    if ((mp[s1[i]] + mp[s2[i]] + flag) % 2 == 1) s3[ans - 2] += 13;
    flag = (mp[s1[i]] + mp[s2[i]] + flag) / 26;
    if (i == 0 && flag > 0) s3[ans - 1] += 13;
  }
  for (int i = n - 1; i >= 0; i--) cout << s3[i];
  return 0;
}
