#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7, mod2 = 998244353;
unsigned long long base = 131;
const int w = 1e7 + 5;
long long m, n;
int main() {
  long long i, j, k, t, qqq;
  cin >> n;
  string a, b;
  cin >> a >> b;
  int flag = 0;
  for (i = a.size() - 1; i >= 0; i--) {
    int x = (b[i] - 'a');
    int y = (int)a[i] + x;
    if (flag) y++, flag = 0;
    if (y > (int)'z') y -= 26, flag++;
    a[i] = (char)y;
  }
  if (flag) a.insert(a.begin(), 'b');
  flag = 0;
  for (i = 0; i < a.size(); i++) {
    int x = a[i] - 'a';
    if (flag) x += 26, flag = 0;
    int y = x % 2;
    x /= 2;
    a[i] = x + 'a';
    if (y) flag = 1;
  }
  if (a.size() == n)
    cout << a << endl;
  else {
    for (i = 1; i < a.size(); i++) printf("%c", a[i]);
    cout << endl;
  }
  return 0;
}
