#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 50;
char s[N], t[N];
int n, a[N], b[N], c[N];
int main(void) {
  scanf("%d", &n);
  scanf("%s", s);
  scanf("%s", t);
  for (int i = 0; i < n; i++) {
    a[i] = s[i] - 'a';
    b[i] = t[i] - 'a';
  }
  for (int i = 0; i < n; i++) {
    c[i] += (a[i] + b[i]) / 2;
    if ((a[i] + b[i]) % 2) {
      c[i + 1] += 13;
    }
  }
  for (int i = n - 1; i >= 0; i--) {
    c[i - 1] += c[i] / 26;
    c[i] %= 26;
  }
  for (int i = 0; i < n; i++) {
    printf("%c", (char)('a' + c[i]));
  }
  printf("\n");
  return 0;
}
