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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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 5
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 1
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 10
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 30
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 1
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#include <bits/stdc++.h>
char ch[35][25];
int visit[1000];
int main() {
  int i, j, n, sum, len;
  while (~scanf("%d", &n)) {
    memset(visit, 0, sizeof(visit));
    getchar();
    for (i = 0; i < n; i++) gets(ch[i]);
    for (i = 0; i < n; i++) {
      len = strlen(ch[i]);
      for (j = 0; j < len; j++) visit[ch[i][j] - 'a'] = 1;
      if (2 == len) {
        sum = 1;
        sum = 27 * (ch[i][0] - 'a') + ch[i][1] - 'a';
        visit[sum] = 1;
      }
    }
    for (i = 0; i < 26; i++) {
      if (0 == visit[i]) {
        printf("%c\n", i + 'a');
        break;
      }
    }
    if (i == 26) {
      for (i = 28; i < 1000; i++) {
        if (0 == visit[i]) {
          printf("%c%c\n", i / 27 + 'a' - 1, i % 27 + 'a');
          break;
        }
      }
    }
  }
  return 0;
}
