#include <bits/stdc++.h>
using namespace std;
vector<int> add(vector<int> a, vector<int> b, int base) {
  reverse(a.begin(), a.end());
  reverse(b.begin(), b.end());
  int carry = 0;
  vector<int> c;
  int l = max(a.size(), b.size());
  for (int i = 0; i < l; i++) {
    int s = (i < a.size() ? a[i] : 0) + (i < b.size() ? b[i] : 0) + carry;
    c.push_back(s % base);
    carry = s / base;
  }
  if (carry) c.push_back(carry);
  reverse(c.begin(), c.end());
  return c;
}
vector<int> half(vector<int> a, int base) {
  vector<int> b;
  int carry = 0;
  for (int i = 0; i < a.size(); i++) {
    int s = base * carry + a[i];
    b.push_back(s / 2);
    carry = s % 2;
  }
  return b;
}
vector<int> avg(vector<int> a, vector<int> b, int base) {
  return half(add(a, b, base), base);
}
int k;
string s, t;
int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> k;
  cin >> s;
  cin >> t;
  vector<int> a, b;
  for (char c : s) {
    a.push_back(c - 'a');
  }
  for (char c : t) {
    b.push_back(c - 'a');
  }
  vector<int> aux = avg(a, b, 26);
  aux.erase(aux.begin(), aux.end() - k);
  string ans;
  for (int n : aux) {
    ans += n + 'a';
  }
  cout << ans << '\n';
}
