#include <bits/stdc++.h>
using namespace std;
vector<int> Convert(string x) {
  vector<int> ans;
  for (int i = 0; i < x.size(); ++i) ans.push_back(x[i] - 'a');
  return ans;
}
vector<int> Subtract(vector<int> x, vector<int> y) {
  int carry = 0;
  vector<int> ans;
  for (int i = x.size() - 1; i >= 0; --i) {
    ans.push_back(x[i] - y[i] - carry);
    carry = 0;
    if (ans.back() < 0) {
      ans.back() += 26;
      carry = 1;
    }
  }
  reverse(ans.begin(), ans.end());
  return ans;
}
vector<int> Add(vector<int> x, vector<int> y) {
  vector<int> ans;
  int carry = 0;
  for (int i = x.size() - 1; i >= 0; --i) {
    ans.push_back(x[i] + y[i] + carry);
    carry = ans.back() / 26;
    ans.back() %= 26;
  }
  reverse(ans.begin(), ans.end());
  return ans;
}
vector<int> Div(vector<int> x) {
  vector<int> ans;
  int carry = 0;
  for (int i = 0; i < x.size(); ++i) {
    ans.push_back(x[i] / 2 + carry * 13);
    carry = x[i] % 2;
  }
  return ans;
}
int main() {
  int k;
  string s1, s2;
  cin >> k;
  cin >> s1 >> s2;
  vector<int> num1 = Convert(s1), num2 = Convert(s2);
  vector<int> ans = Add(num1, Div(Subtract(num2, num1)));
  for (int i = 0; i < ans.size(); ++i) cout << (char)(ans[i] + 'a');
  return 0;
}
