#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-8;
const double pi = acos(-1);
const long long inf = 0x3f3f3f3f3f3f3f3f;
const int INF = 0x3f3f3f3f;
const int MAX = 2e5 + 10;
const long long mod = 1e9 + 7;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
int k;
string s, t;
int c[MAX * 2];
int ans[MAX * 2];
int main() {
  cin >> k >> s >> t;
  for (int i = (0); i < (k); ++i) {
    int x = s[k - i - 1] - 'a' + t[k - i - 1] - 'a';
    c[i] += x;
    if (c[i] >= 26) c[i + 1]++, c[i] -= 26;
  }
  int mk = k;
  if (c[k] != 0) mk++;
  for (int i = mk - 1; i >= 0; i--) {
    ans[i] = c[i] / 2;
    if (i > 0 && c[i] % 2 != 0) c[i - 1] += 26;
  }
  for (int i = mk - 1; i >= 0; i--) {
    if (i == k && ans[i] == 0) continue;
    char C = ans[i] + 'a';
    cout << C;
  }
}
