#include <bits/stdc++.h>
#pragma warning(disable : 4996)
#pragma comment(linker, "/STACK:336777216")
using namespace std;
inline long long gcd(long long a, long long b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
inline long long power(long long a, long long n, long long m) {
  if (n == 0) return 1;
  long long p = power(a, n / 2, m);
  p = (p * p) % m;
  if (n % 2)
    return (p * a) % m;
  else
    return p;
}
const long long MOD = 1000000007;
const long long int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3f;
int32_t main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long int k;
  cin >> k;
  string s, t;
  cin >> s >> t;
  long long int digit[k + 1];
  long long int cr = 0, sum, ch, vl;
  for (long long int i = k; i >= 1; i--) {
    sum = s[i - 1] - 'a' + t[i - 1] - 'a' + cr;
    digit[i] = sum % 26;
    cr = sum / 26;
  }
  if (cr) digit[1] += 26;
  for (long long int i = 1; i <= k; i++) {
    if ((digit[i] % 2) == 0) {
      ch = digit[i] / 2;
      cout << (char)(ch + 97);
    } else {
      ch = (digit[i] - 1) / 2;
      digit[i + 1] += 26;
      cout << (char)(ch + 97);
    }
  }
  cout << "\n";
  return 0;
}
