#include <bits/stdc++.h>
using namespace std;
const int maxn = 300300;
int a[maxn];
int main(void) {
  string s1, s2, ss = "", str;
  int n, i, j, x, y, tp = 0, z;
  cin >> n;
  cin >> s1 >> s2;
  for (i = n - 1; i >= 0; i--) {
    x = s1[i] - 'a';
    y = s2[i] - 'a';
    z = (y - x);
    if (z < 0) {
      s2[i - 1]--;
      a[i] = (z + 26);
    } else
      a[i] = z;
  }
  for (i = 0; i < n; i++) {
    if (a[i] % 2 == 0)
      a[i] = a[i] / 2;
    else {
      a[i + 1] += 26;
      a[i] = a[i] / 2;
    }
  }
  for (i = n - 1; i >= 0; i--) {
    x = (s1[i] - 'a') + a[i];
    if (x < 26)
      s1[i] = (char)(x + 'a');
    else {
      s1[i - 1]++;
      s1[i] = (char)(x % 26 + 'a');
    }
  }
  for (i = 0; i < n; i++) cout << s1[i];
  cout << endl;
  return 0;
}
