#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  string s, t, ans = "";
  cin >> n >> s >> t;
  int pre = 0;
  for (int i = n - 1; i >= 0; i--) {
    int sum = s[i] - 'a' + t[i] - 'a' + pre;
    ans = (char)(sum % 26 + 'a') + ans;
    pre = sum / 26;
  }
  for (int i = 0; i < n; i++) {
    int sum = pre * 26 + ans[i] - 'a';
    cout << (char)(sum / 2 + 'a');
    pre = sum % 2;
  }
  cout << endl;
  return 0;
}
