#include <bits/stdc++.h>
using namespace std;
const int MAXN = 200005;
int len;
char s1[MAXN];
char s2[MAXN];
int de[MAXN];
int main() {
  scanf("%d", &len);
  scanf("%s", s1);
  scanf("%s", s2);
  for (int i = 0; i < len; ++i) {
    de[i] = s2[i] - s1[i];
  }
  for (int i = len - 1; i > 0; --i) {
    if (de[i] < 0) {
      de[i - 1] -= 1;
      de[i] += 26;
    }
  }
  for (int i = 0; i < len; ++i) {
    int p = de[i] % 2;
    de[i] /= 2;
    if (p == 1) de[i + 1] += 26;
  }
  for (int i = len - 1; i > 0; --i) {
    if (s1[i] + de[i] > 'z') {
      s1[i] -= 26;
      s1[i] += de[i];
      s1[i - 1] += 1;
    } else
      s1[i] += de[i];
  }
  s1[0] += de[0];
  for (int i = 0; i < len; ++i) {
    cout << (char)s1[i];
  }
}
