#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
int n;
string median(string second, string t) {
  string z;
  for (int i = 0; i <= n - 1; i++) z += ' ';
  int carry = 0;
  for (int i = n - 1; i >= 0; i--) {
    int curr = (second[i] - 'a') + (t[i] - 'a') + carry;
    if (curr >= 26) {
      curr -= 26;
      carry = 1;
    } else {
      carry = 0;
    }
    z[i] = curr + 'a';
  }
  if (carry) {
    z = 'b' + z;
  }
  int sub = 0;
  string y = "";
  for (int i = 0; i <= z.size() - 1; i++) {
    int curr = (z[i] - 'a' + (sub ? 26 : 0)) / 2;
    sub = (z[i] - 'a' + (sub ? 26 : 0)) % 2;
    char newch = 'a' + curr;
    y += newch;
  }
  if (y.size() > second.size()) {
    string y2 = "";
    for (int i = 1; i <= y.size() - 1; i++) {
      y2 += y[i];
    }
    return y2;
  }
  return y;
}
int main() {
  scanf("%d", &n);
  string second, t, z;
  cin >> second >> t;
  z = median(second, t);
  cout << z << endl;
  return 0;
}
