#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
int main() {
  int N; cin >> N;
  vector<string> A(N);
  for(int i = 0; i < N; i++) cin >> A[i];
  string ans = "";
  map<string,int> dic;
  for(int j = 0; j < N; j++) {
    int a = 0;
    int b = 0;
    for(int i = 0; i < N; i++) {
      if(A[i][j] == 'a') {
        a++;
      }else {
        b++;
      }
    }
    if(a >= b) {
      ans += 'b';
    }else {
      ans += 'a';
    }
    dic[A[j]]++;
  }
  
  if(dic[ans] > 0) {
    cout << "aba" << endl;
    return 0;
  }
  cout << ans << endl;
}