#include <bits/stdc++.h>
using namespace std;
int c[200010];
int main() {
  int k, m, n;
  cin >> k;
  string x, y;
  cin >> x >> y;
  for (int i = 0; i < k; i++) {
    m = x[k - i - 1] - 'a';
    n = y[k - i - 1] - 'a';
    c[i] += m + n;
    c[i + 1] += c[i] / 26;
    c[i] %= 26;
  }
  for (int i = k - 1; i >= 0; i--) {
    c[i] += c[i + 1] % 2 * 26;
    c[i + 1] /= 2;
  }
  c[0] = c[0] / 2;
  for (int i = k - 1; i >= 0; i--) printf("%c", c[i] + 'a');
  return 0;
}
