3
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 1
z
 30
b
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p
n
f
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k
p
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a
c
w
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z
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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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skrvnyz
avptgpjmenfzfxwckmt
 30
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 1
b

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

#include <bits/stdc++.h>
int main() {
  int n, l[30], i, j, p;
  char s[30][22], a, b, c;
  while (scanf("%d", &n) != EOF) {
    for (i = 0; i < n; i++) {
      scanf("%s", s[i]);
      l[i] = strlen(s[i]);
    }
    for (a = 97; a <= 122; a++) {
      p = 0;
      for (i = 0; i < n; i++) {
        for (j = 0; j < l[i]; j++)
          if (s[i][j] == a) {
            p = 1;
            break;
          }
        if (p == 1) break;
      }
      if (p == 0) {
        printf("%c\n", a);
        p = 2;
        break;
      }
    }
    if (p != 2) {
      for (a = 97; a <= 122; a++) {
        c = 123;
        for (i = 0; i < n; i++) {
          for (j = 0; j < l[i]; j++) {
            if (s[i][j] == a && j != l[i] - 1) {
              for (b = 97; b <= 122; b++) {
                if (s[i][j + 1] != b) {
                  if (c > b) {
                    c = b;
                    p = 2;
                  }
                }
              }
            }
          }
        }
        if (p = 2) {
          printf("%c%c\n", a, c);
          break;
        }
      }
    }
  }
  return 0;
}
