3
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 1
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 30
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 5
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 2
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 5
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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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 30
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 1
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a
 a
 e
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 aa
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 ab
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 a

#include <bits/stdc++.h>
int a[26];
int substr(char sub[], char str[]) {
  int i, j, k, l, a;
  k = strlen(sub);
  l = strlen(str);
  for (i = 0; i < l - k + 1; i++) {
    a = 0;
    for (j = 0; j < k; j++)
      if (sub[j] == str[i + j]) a++;
    if (a == k) return 1;
  }
  return 0;
}
int main() {
  int n, i, j, k, flag = 0, f;
  char s[30][25], sub[3];
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    scanf("%s", s[i]);
    j = strlen(s[i]);
    for (k = 0; k < j; k++) {
      a[s[i][k] - 97]++;
    }
  }
  for (i = 0; i < 26; i++) {
    if (a[i] == 0) {
      printf("%c\n", i + 97);
      flag = 1;
      break;
    }
  }
  if (flag == 0) {
    for (i = 0; i < 26; i++) {
      for (j = 0; j < 26; j++) {
        f = 0;
        sub[0] = i + 97;
        sub[1] = j + 97;
        sub[2] = '\0';
        for (k = 0; k < n; k++) {
          if (substr(sub, s[k]))
            break;
          else
            f++;
        }
        if (f == n) break;
      }
      if (f == n) break;
    }
    printf("%s\n", sub);
  }
  return 0;
}
