#include <bits/stdc++.h>
const double PI = acos(-1.0);
const int maxn = 1e6;
const int INF = 1e9;
const long long linf = 0x3f3f3f3f3f3f3f3f;
using namespace std;
int a[maxn];
char s[maxn], t[maxn];
int main() {
  int len;
  while (cin >> len) {
    scanf("%s%s", s + 1, t + 1);
    memset(a, 0, sizeof a);
    for (int i = 1; i <= len; i++) a[i] = (s[i] - 'a') + (t[i] - 'a');
    for (int i = len; i >= 2; i--) {
      a[i - 1] += a[i] / 26;
      a[i] %= 26;
    }
    for (int i = 1; i <= len; i++) {
      if (a[i] & 1) a[i + 1] += 26;
      printf("%c", 'a' + a[i] / 2);
    }
    cout << endl;
  }
  return 0;
}
