#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <map>
#include <set>
#include <queue>
#include <iomanip>
#include <cstring>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i,n) for (int i = 0; i < int(n);i++)

int main(){
  int n;
  cin >> n;
  vector<string> a(n);
  for (int i = 0; i < n;i++) cin >> a[i];
  sort(a.begin(),a.end());
  string ans;
  if (n == 1 && a[0][0] == 'a') ans = 'b';
  else if (n == 1 && a[0][0] == 'b') ans = 'a';

  for (int i = 0; i < n-1;i++){
    string ns = a[i];
    ns[n-1]++;
    for (int j = n-1; j >= 1;j--){
      if (ns[j] == 'c'){
        ns[j+1]++;
        ns[j]='a';
      }
    }
    if (ns[0] == 'c') continue;
    ans = ns;
  }
  cout << ans << endl;


  return 0;
}