#include <bits/stdc++.h>
using namespace std;
mt19937 rng32(chrono::steady_clock::now().time_since_epoch().count());
long long modExpo(long long x, long long n) {
  if (n == 0)
    return 1;
  else if (n % 2 == 0)
    return modExpo((x * x) % 998244353, n / 2);
  else
    return (x * modExpo((x * x) % 998244353, (n - 1) / 2)) % 998244353;
}
long long modInverse(long long n) { return modExpo(n, 1000000007 - 2); }
long long n, m, k, ar[200010], pos, vis[200010];
string s, s1;
vector<long long> adj[200010];
vector<pair<long long, long long> > edge;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int ti = 1;
  while (ti--) {
    string t = "abcdefghijklmnopqrstuvwxyz";
    cin >> n;
    cin >> s >> s1;
    string s2;
    s2.resize(n + 1);
    reverse(s.begin(), s.end());
    reverse(s1.begin(), s1.end());
    int last = 0;
    for (long long i = 0; i < n; i++) {
      int temp = s[i] - 'a' + s1[i] - 'a' + last;
      s2[i] = t[temp % 26];
      last = temp / 26;
    }
    s2[n] = t[last];
    int carry = 0;
    for (long long i = n; i >= 0; i--) {
      int x = s2[i] - 'a' + 26 * carry;
      s2[i] = t[x / 2];
      carry = x % 2;
    }
    reverse(s2.begin(), s2.end());
    cout << s2.substr(1, n);
  }
  return 0;
}
