#include <bits/stdc++.h>
using namespace std;
int s1[200005], s2[200005], soma[200005], n;
char c;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> c;
    s1[i] = c - 'a';
  }
  for (int i = 1; i <= n; i++) {
    cin >> c;
    s2[i] = c - 'a';
  }
  int carry = 0;
  for (int i = n; i >= 1; i--) {
    int s = s1[i] + s2[i] + carry;
    soma[i] = s % 26;
    carry = s / 26;
  }
  soma[0] = -1;
  if (carry > 0) soma[0] = carry;
  carry = 0;
  for (int i = n; i >= 1 || (i == 0 && soma[0] != -1); i--) {
    if (soma[i] % 2 == 0)
      soma[i] /= 2;
    else {
      int m1 = soma[i] - 1;
      soma[i + 1] += 13;
      int extra = soma[i + 1] >= 26 ? soma[i + 1] / 26 : 0;
      soma[i + 1] %= 26;
      soma[i] = extra + m1 / 2;
    }
  }
  for (int i = 1; i <= n; i++) {
    cout << (char)('a' + soma[i]) << (i == n ? "\n" : "");
  }
}
