#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 10;
char s1[maxn], s2[maxn];
int q1[maxn], q2[maxn], q3[maxn];
int main() {
  int n;
  cin >> n;
  scanf("%s %s", s1 + 1, s2 + 1);
  for (int i = 1; i <= n; i++) q1[i] = s1[i] - 'a', q2[i] = s2[i] - 'a';
  reverse(q1 + 1, q1 + n + 1);
  reverse(q2 + 1, q2 + n + 1);
  int up = n;
  for (int i = 1; i <= up; i++) {
    q3[i] += q1[i] + q2[i];
    q3[i + 1] += q3[i] / 26;
    q3[i] %= 26;
    if (q3[up + 1] != 0) up++;
  }
  int flag;
  if (q3[1] % 2 == 1)
    flag = 1;
  else
    flag = 0;
  for (int i = up; i >= 1; i--) {
    if (q3[i] % 2 == 0)
      q3[i] /= 2;
    else {
      q3[i - 1] += 26;
      q3[i] /= 2;
    }
  }
  if (flag) q3[1]++;
  while (q3[up] == 0 && up > n) up--;
  for (int i = up; i >= 1; i--) {
    char temp;
    temp = 'a' + q3[i];
    cout << temp;
  }
  return 0;
}
