#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
int k, a[N], b[N];
char s[N], t[N];
int main() {
  cin >> k;
  scanf("%s", s + 1);
  scanf("%s", t + 1);
  for (int i = k; i; i--) {
    a[i] = t[i] - s[i];
    if (a[i] < 0) {
      a[i] += 26;
      t[i - 1]--;
    }
  }
  for (int i = 1; i <= k; i++) {
    a[i + 1] += (a[i] % 2) * 26;
    a[i] /= 2;
  }
  for (int i = k; i; i--) {
    b[i] += s[i] - 'a' + a[i];
    b[i - 1] += b[i] / 26;
    b[i] %= 26;
  }
  for (int i = 1; i <= k; i++) printf("%c", 'a' + b[i]);
  return 0;
}
