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