#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-11;
const int maxn = 2e5 + 9;
const double pi = 3.14159265358979323846264;
const int inf = 2e9;
const int mod = 998244353;
char a[maxn], b[maxn];
int op[maxn];
int main() {
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  cin >> a >> b;
  int tmp = 0, num = 0;
  for (int i = n - 1; i >= 0; --i) {
    int p = a[i] - 'a';
    int q = b[i] - 'a';
    op[num++] = (p + q + tmp) % 26;
    tmp = (p + q + tmp) / 26;
  }
  if (tmp) op[num++] = tmp;
  vector<int> pq;
  pq.clear();
  tmp = 0;
  for (int i = num - 1; i >= 0; --i) {
    int jk = (tmp + op[i]) / 2;
    pq.push_back(jk);
    tmp = (tmp + op[i]) % 2 * 26;
  }
  for (int i = pq.size() - n; i < pq.size(); ++i) {
    cout << char(pq[i] + 'a');
  }
  return 0;
}
