#include <bits/stdc++.h>
using namespace std;
const int MOD = 998244353;
const int MAX = 2e5 + 10;
int k;
string S, T;
vector<int> ADD(vector<int> A, vector<int> B) {
  for (int i = k; i >= 0; i--) {
    A[i] += B[i];
    if (i > 0) {
      A[i - 1] += A[i] / 26;
      A[i] %= 26;
    }
  }
  return A;
}
vector<int> Division(vector<int> A, int d) {
  for (int i = 0; i <= k; i++) {
    int r = A[i] % d;
    A[i] /= d;
    if (i < k) A[i + 1] += r * 26;
  }
  return A;
}
int main() {
  cin >> k >> S >> T;
  vector<int> A(k + 1), B(k + 1);
  for (int i = 1; i <= k; i++) {
    A[i] = S[i - 1] - 'a';
  }
  for (int i = 1; i <= k; i++) {
    B[i] = (T[i - 1] - 'a');
  }
  A = ADD(A, B);
  A = Division(A, 2);
  for (int i = 1; i <= k; i++) {
    cout << (char)(A[i] + 'a');
  }
  return 0;
}
