#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  scanf("%d", &k);
  string a, b, s, r;
  s.resize(k + 3, 0);
  cin >> a;
  cin >> b;
  for (int i = 0; i < k; i++) {
    a[i] -= 'a';
    b[i] -= 'a';
  }
  int q = 0;
  for (int i = k - 1; i >= 0; i--) {
    s[i + 1] = (q + a[i] + b[i]) % 26;
    q = (q + a[i] + b[i]) > 25;
    s[i] = q;
  }
  int resto = 0;
  for (int i = 0; i < k + 1; i++) {
    r.push_back((s[i] + 26 * resto) / 2);
    resto = (s[i] + 26 * resto) % 2;
  }
  for (int i = 1; i <= k; i++) {
    printf("%c", r[i] + 'a');
  }
  printf("\n");
  return 0;
}
