#include <bits/stdc++.h>
using namespace std;
int k;
vector<int> convert(string &S) {
  vector<int> ans(k + 1);
  ans[0] = 0;
  for (int i = 0; i < k; i++) {
    ans[i + 1] = S[i] - 'a';
  }
  return ans;
}
vector<int> getMedian(vector<int> S, vector<int> T) {
  for (int i = k; i >= 0; i--) {
    S[i] += T[i];
    if (i) {
      S[i - 1] += S[i] / 26;
      S[i] %= 26;
    }
  }
  for (int i = 0; i <= k; i++) {
    int rem = S[i] % 2;
    S[i] /= 2;
    if (i + 1 <= k) {
      S[i + 1] += rem * 26;
    }
  }
  return S;
}
int main() {
  string s, t;
  cin >> k;
  cin >> s;
  cin >> t;
  vector<int> vecS = convert(s);
  vector<int> vecT = convert(t);
  vector<int> finalVec = getMedian(vecS, vecT);
  for (int i = 1; i <= k; i++) {
    cout << (char)('a' + finalVec[i]);
  }
  return 0;
}
