#include <bits/stdc++.h>
using namespace std;
char a[220000], b[220000];
int c[220000];
int main() {
  int sum = 0, i, n;
  cin >> n;
  cin >> a >> b;
  memset(c, 0, sizeof(c));
  for (i = 0; i < n; i++) c[i] = (a[i] - 'a' + b[i] - 'a');
  for (i = n - 1; i >= 1; i--) {
    c[i - 1] = c[i - 1] + c[i] / 26;
    c[i] = c[i] % 26;
  }
  for (i = 0; i < n; i++) {
    if (c[i] % 2 != 0) c[i + 1] = c[i + 1] + 26;
    putchar('a' + c[i] / 2);
  }
}
