#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n;
  string s, t;
  cin >> n >> s >> t;
  string sum = "";
  long long int carry = 0;
  for (long long int i = n - 1; i >= 0; i--) {
    long long int d = (s[i] - 'a') + (t[i] - 'a') + carry;
    sum += i ? (d % 26) : d;
    carry = d / 26;
  }
  reverse(sum.begin(), sum.end());
  carry = 0;
  string ans = "";
  for (long long int i = 0; i < n; i++) {
    long long int d = carry * 26 + sum[i];
    ans += (d / 2) + 'a';
    carry = d % 2;
  }
  cout << ans << endl;
}
