#include <bits/stdc++.h>
using namespace std;
int ans[300100];
int main() {
  int n;
  cin >> n;
  string a, b;
  cin >> a >> b;
  int c = 0;
  for (int i = n - 1; i >= 0; i--) {
    int x = (a[i] + b[i] - 2 * ('a')) + c;
    ans[i] = x % 26;
    c = x / 26;
  }
  for (int i = 0; i < n; i++) {
    int x = ans[i] + c * 26;
    cout << char('a' + x / 2);
    c = x % 2;
  }
}
