#include <bits/stdc++.h>
using namespace std;
int a[200005];
int b[200005];
int c[200005];
char str1[200005];
char str2[200005];
int main() {
  int n;
  while (scanf("%d", &n) != EOF) {
    cin >> str1 >> str2;
    int i;
    for (i = 0; i < n; i++) {
      a[i] = str1[i] - 'a';
      b[i] = str2[i] - 'a';
    }
    for (i = n - 1; i >= 0; i--) {
      if (b[i] >= a[i]) {
        c[i] = b[i] - a[i];
      } else {
        c[i] = b[i] - a[i] + 26;
        b[i - 1]--;
      }
    }
    for (i = 0; i < n; i++) {
      if (c[i] % 2 == 0) {
        c[i] = c[i] / 2;
      } else {
        c[i] = c[i] / 2;
        c[i + 1] += 26;
      }
    }
    for (i = n - 1; i >= 0; i--) {
      a[i] = a[i] + c[i];
      while (a[i] >= 26) {
        a[i] -= 26;
        a[i - 1]++;
      }
    }
    for (i = 0; i < n; i++) {
      printf("%c", (char)(a[i] + 'a'));
    }
    printf("\n");
  }
  return 0;
}
