#include <bits/stdc++.h>
using namespace std;
long long MOD = pow(10, 9) + 7;
long long INF = 1e18 + 10;
int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  string t;
  cin >> t;
  int ss[k], tt[k];
  for (int i = 0; i < k; i++) {
    ss[i] = s[i] - 'a' + 1;
    tt[i] = t[i] - 'a' + 1;
  }
  int dif[k];
  for (int i = k - 1; i >= 0; i--) {
    if (tt[i] >= ss[i])
      dif[i] = (tt[i] - ss[i]);
    else
      dif[i] = (tt[i] + 26 - ss[i]), tt[i - 1] -= 1;
  }
  for (int i = 0; i < k; i++) {
    if (dif[i] & 1) {
      dif[i] /= 2;
      dif[i + 1] += 26;
    } else {
      dif[i] /= 2;
    }
  }
  int cat[k];
  memset(cat, 0, sizeof cat);
  for (int i = k - 1; i >= 0; i--) {
    cat[i] += (ss[i] + dif[i]);
    if (cat[i] > 26) {
      cat[i] -= 26;
      cat[i - 1] += 1;
    }
  }
  for (int i = 0; i < k; i++) cout << (char)(cat[i] + 'a' - 1);
}
