#include <bits/stdc++.h>
using namespace std;
long long mod(long long a) {
  long long c = a % (long long)(1E9 + 7);
  if (c >= 0) return c;
  return c + (long long)(1E9 + 7);
}
long long inf = (long long)1E17;
long long max(long long a, long long b) {
  if (a < b)
    return b;
  else
    return a;
}
long long min(long long a, long long b) {
  if (a > b)
    return b;
  else
    return a;
}
void Max(long long &pos, long long val) { pos = max(pos, val); }
void Min(long long &pos, long long val) { pos = min(pos, val); }
void Add(long long &pos, long long val) { pos = mod(pos + val); }
long double EPS = 1E-9;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long k;
  cin >> k;
  string s, t;
  cin >> s >> t;
  long long a[k], b[k];
  for (long long i = 0; i <= k - 1; i++) {
    a[i] = (s[i] - 'a');
    b[i] = (t[i] - 'a');
  }
  long long sum[k];
  for (long long i = 0; i <= k - 1; i++) {
    sum[i] = a[i] + b[i];
  }
  for (long long i = k - 1; i >= 1; i--) {
    long long res = sum[i] / 26;
    sum[i] -= 26 * res;
    sum[i - 1] += res;
  }
  string ans;
  long long res = 0;
  for (long long i = 0; i <= k - 1; i++) {
    res *= 26;
    res += sum[i];
    ans += ('a' + res / 2);
    res %= 2;
  }
  cout << ans << endl;
  return 0;
}
