#include <bits/stdc++.h>
using namespace std;
long long ansp[200001];
int main() {
  long long k;
  memset(ansp, 0, sizeof(ansp));
  scanf("%lld", &k);
  char s[200001], t[200001], ans[200001];
  scanf("%s %s", s, t);
  long long add = 0, dec = 0, again = 0;
  long long i, j;
  for (i = 0; i < k; i++) {
    long long x = s[i] - 'a', y = t[i] - 'a' + add;
    add = 0;
    if ((x + y) % 2 != 0) {
      add = 26;
      y--;
    }
    long long now = (x + y) / 2;
    ansp[i] = now;
  }
  for (i = k - 1; i >= 0; i--) {
    ansp[i - 1] += ansp[i] / 26;
    ansp[i] %= 26;
  }
  for (i = 0; i < k; i++) printf("%c", (char)(ansp[i] + 'a'));
}
