#include <bits/stdc++.h>
using namespace std;
const int mx = (int)2e5 + 5;
int n, c[mx], ans[mx];
char a[mx], b[mx];
void solve() {
  int cnt = 0, temp = 0;
  for (int i = n - 1; i >= 0; i--) {
    c[cnt++] = (a[i] + b[i] - 2 * 'a' + temp) % 26;
    temp = (a[i] + b[i] - 2 * 'a' + temp) / 26;
  }
  c[cnt++] = temp;
  temp = 0;
  for (int i = cnt - 1; i >= 0; i--) {
    temp *= 26;
    ans[i] = (c[i] + temp) / 2;
    temp = (c[i] + temp) % 2;
  }
  for (int i = n - 1; i >= 0; i--) {
    printf("%c", ans[i] + 'a');
  }
  cout << endl;
}
int main() {
  cin >> n;
  scanf("%s%s", a, b);
  solve();
}
