#include <bits/stdc++.h>
using namespace std;
const int maxm = 300005;
int main() {
  int n;
  char a[maxm], b[maxm];
  cin >> n;
  cin >> a >> b;
  int c[maxm] = {0};
  int fg = 0;
  for (int i = n - 1; i >= 0; i--) {
    int s = (a[i] - 'a') + (b[i] - 'a') + fg;
    if (s >= 26 && i != 0) {
      s -= 26;
      fg = 1;
    } else
      fg = 0;
    c[i] = s;
  }
  for (int i = 0; i < n; i++) {
    if (c[i] % 2) c[i + 1] += 26;
    c[i] /= 2;
  }
  for (int i = 0; i < n; i++) cout << (char)(c[i] + 'a');
  cout << endl;
  return 0;
}
