#include <bits/stdc++.h>
using namespace std;
vector<int> get(const string a) {
  vector<int> res(a.size() + 1);
  for (int i = 0; i < int(a.size()); i++) res[i + 1] = a[i] - 'a';
  return res;
}
int main() {
  int n, k;
  cin >> k;
  string a, b;
  cin >> a >> b;
  vector<int> aa = get(a), bb = get(b);
  for (int i = k; i >= 0; --i) {
    aa[i] += bb[i];
    if (i) {
      int temp = aa[i] / 26;
      aa[i - 1] += temp;
      aa[i] %= 26;
    }
  }
  for (int i = 0; i <= k; i++) {
    int rem = aa[i] % 2;
    aa[i] /= 2;
    if (i + 1 <= k) {
      aa[i + 1] += rem * 26;
    }
  }
  for (int i = 1; i <= k; ++i) cout << char(aa[i] + 'a');
}
