#include <bits/stdc++.h>
using namespace std;
const long long ALPH = 'z' - 'a' + 1;
const int LEN = (int)2e5;
int k;
string s1, s2;
vector<long long> res;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cin >> k >> s1 >> s2;
  res.push_back(0);
  for (int i = k - 1; i >= 0; i--) {
    long long sum =
        (long long)(s1[i] - 'a') + (long long)(s2[i] - 'a') + res.back();
    res.back() = sum % ALPH;
    res.push_back(sum / ALPH);
  }
  if (res.size() > k) {
    res[res.size() - 2] += res.back() * ALPH;
    res.pop_back();
  }
  for (auto i = res.rbegin(); i != res.rend(); i++) {
    *(i + 1) += (*i & 1LL) * ALPH;
    *i >>= 1;
    cout << (char)(*i + 'a');
  }
  return 0;
}
