#include <bits/stdc++.h>
using namespace std;
const int N = 200010;
int n, ans[N];
char s1[N], s2[N];
int main() {
  scanf("%d%s%s", &n, s1 + 1, s2 + 1);
  for (int i = n; i; i--) {
    ans[i] += s1[i] + s2[i] - 2 * 'a';
    ans[i - 1] += ans[i] / 26;
    ans[i] %= 26;
  }
  for (int i = 0; i <= n; i++) {
    if (ans[i] & 1) ans[i + 1] += 26;
    ans[i] /= 2;
  }
  for (int i = 1; i <= n; i++) putchar(ans[i] + 'a');
  return 0;
}
