#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  cin >> k;
  string S, T;
  cin >> S >> T;
  vector<int> m(k, 0);
  for (int i = k - 1; i >= 0; i--) {
    int sum = m[i] + T[i] - S[i];
    if (sum < 0) {
      assert(i > 0);
      sum += 26;
      m[i - 1]--;
    }
    assert(sum >= 0 && sum < 26);
    m[i] = sum;
  }
  int sum = 0;
  for (int i = 0; i < k; i++) {
    int temp = m[i] + sum * 26;
    m[i] = temp / 2;
    sum = temp % 2;
  }
  assert(sum == 0);
  sum = 0;
  for (int i = k - 1; i >= 0; i--) {
    int temp = S[i] + sum + m[i];
    if (temp > 'z') {
      temp -= 26;
      sum = 1;
    } else {
      sum = 0;
    }
    S[i] = temp;
  }
  assert(sum == 0);
  cout << S;
  return 0;
}
