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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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 2
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 1
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 30
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 1
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#include <bits/stdc++.h>
using namespace std;
char temp[100];
char arr[100][100];
int t;
int substr(char a[100], char b[100]) {
  int l1, l2, f, i, j;
  l1 = strlen(a);
  l2 = strlen(b);
  f = 0;
  for (i = 0; i < ((l1 - l2) + 1); i++) {
    f = 0;
    for (j = 0; j < l2; j++) {
      if (a[i + j] != b[j]) {
        f = 1;
        break;
      }
    }
    if (f == 0) return 1;
  }
  return 0;
}
void gen(int i, int n) {
  int j, fl;
  if (i == n) {
    temp[n] = 0;
    fl = 1;
    for (j = 0; j < t; j++) {
      if (substr(arr[j], temp) == 1) {
        fl = 0;
        return;
        printf("%s\n", temp);
        exit(0);
        return;
      }
    }
    if (fl == 1) {
      printf("%s\n", temp);
      exit(0);
    }
  }
  for (j = 97; j < 123; j++) {
    temp[i] = (char)j;
    gen(i + 1, n);
  }
  return;
}
int main() {
  int f, n, i, j, k, a, b, m;
  scanf("%d", &t);
  m = 0;
  for (i = 0; i < t; i++) {
    scanf("%s", arr[i]);
  }
  for (i = 1; i <= 32; i++) {
    gen(0, i);
  }
  return 0;
}
