#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  cin >> n;
  string s1, s2;
  cin >> s1 >> s2;
  long long ar[n];
  long long carry = 0;
  for (long long i = n - 1; i >= 0; i--) {
    ar[i] = (s1[i] + s2[i] - 'a' - 'a' + carry);
    if (i != 0) {
      carry = ar[i] / 26;
      ar[i] %= 26;
    }
  }
  long long ans[n];
  carry = 0;
  for (long long int i = 0; i < n; i++) {
    ans[i] = (ar[i] + carry) / 2;
    if ((ar[i] + carry) % 2)
      carry = 26;
    else
      carry = 0;
  }
  for (long long int i = 0; i < n; i++) cout << char(ans[i] + 'a');
  return 0;
}
