#include <bits/stdc++.h>
using namespace std;
const int maxn = 2 * 100000 + 5;
char str1[maxn];
char str2[maxn];
int ans[maxn];
int n;
int add() {
  int carry = 0;
  for (int i = n - 1, temp; i >= 0; i--) {
    temp = (str1[i] - 'a') + (str2[i] - 'a') + carry;
    ans[i] = temp % 26;
    carry = temp / 26;
  }
  return carry;
}
void div(int carry) {
  for (int i = 0, temp; i < n; i++) {
    temp = ans[i] + 26 * carry;
    ans[i] = temp / 2;
    carry = temp % 2;
  }
}
int main() {
  scanf("%d", &n);
  scanf("%s", str1);
  scanf("%s", str2);
  int carry = add();
  div(carry);
  for (int i = 0; i < n; i++) {
    printf("%c", ans[i] + 'a');
  }
  printf("\n");
  return 0;
}
