#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  cin >> k;
  string s, t;
  cin >> s >> t;
  int a[k + 1], b[k + 1];
  memset(a, 0, sizeof(a));
  memset(b, 0, sizeof(b));
  for (int i = 0; i < k; i++) {
    a[i + 1] = (s[i] - 'a');
    b[i + 1] = (t[i] - 'a');
  }
  for (int i = k; i >= 0; i--) {
    a[i] = a[i] + b[i];
    if (i > 0) {
      int carry = a[i] / 26;
      a[i - 1] += carry;
      a[i] %= 26;
    }
  }
  for (int i = 0; i <= k; i++) {
    if (i < k) {
      int remainder = a[i] % 2;
      a[i + 1] += (remainder * 26);
    }
    a[i] /= 2;
  }
  for (int i = 1; i <= k; i++) cout << (char)(a[i] + 'a');
  return 0;
}
