3
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 1
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 30
b
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k
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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xpbtbpftimnn
khbknqbvdkdd
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avptgpjmenfzfxwckmt
 30
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 1
b

a
 a
 e
 a
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 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
int N;
char ar[35][30];
int yol[35];
struct node {
  int cnt;
  struct node *s[30];
} * root;
void tree(int z, int x, int y, node *k) {
  if (x == y + 1) {
    k->cnt = 1;
    return;
  }
  if (!k->s[ar[z][x] - 'a'])
    k->s[ar[z][x] - 'a'] = (node *)calloc(1, sizeof(node));
  tree(z, x + 1, y, k->s[ar[z][x] - 'a']);
}
void yaz(node *k, int t) {
  for (int i = 0; i <= 25; i++)
    if (!k->s[i]) {
      for (int j = 1; j < t; j++) printf("%c", yol[j] + 'a');
      printf("%c", i + 'a');
      exit(0);
    }
  for (int i = 0; i <= 25; i++) {
    yol[t] = i;
    yaz(k->s[i], t + 1);
  }
}
int main() {
  cin >> N;
  root = (node *)calloc(1, sizeof(node));
  for (int i = 1; i <= N; i++) {
    scanf(" %s", ar[i] + 1);
    for (int j = 1; j <= strlen(ar[i] + 1); j++)
      for (int k = j; k <= strlen(ar[i] + 1); k++) tree(i, j, k, root);
  }
  yaz(root, 1);
  return 0;
}
