#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int k;
  cin >> k;
  string s, t;
  cin >> s;
  cin >> t;
  int sarr[k];
  int tarr[k];
  for (int i = k - 1; i >= 0; i--) {
    sarr[i] = s[i] - 'a';
    tarr[i] = t[i] - 'a';
  }
  int sumarr[k + 1];
  int carry = 0;
  for (int i = k - 1; i >= 0; i--) {
    sumarr[i + 1] = (sarr[i] + tarr[i] + carry) % 26;
    if (sarr[i] + tarr[i] + carry >= 26) {
      carry = 1;
    } else {
      carry = 0;
    }
  }
  sumarr[0] = carry;
  for (int i = 0; i <= k; i++) {
    if (sumarr[i] % 2 == 1) {
      sumarr[i + 1] += 26;
    }
    sumarr[i] /= 2;
  }
  for (int i = 1; i <= k; i++) {
    cout << (char)(sumarr[i] + 'a');
  }
  cout << endl;
}
