3
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 1
z
 30
b
u
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k
p
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a
c
w
z
z
p
t
d
o
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y
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 5
splt
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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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abcdefghijklmnopqrst
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 1
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 10
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 5
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abcdefghijklmnopqrst
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 2
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txwdjmohrrszorrnomc
 1
a
 30
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skrvnyz
avptgpjmenfzfxwckmt
 30
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 1
b

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

#include <bits/stdc++.h>
using namespace std;
int N;
vector<string> vec;
int main(int argc, char *argv[]) {
  int N;
  cin >> N;
  vec.resize(N);
  for (int i = 0, _i = N; i < _i; ++i) {
    cin >> vec[i];
  }
  for (char i = 'a'; i <= 'z'; i++) {
    bool good = true;
    for (int j = 0, _j = N; j < _j; ++j) {
      good &= vec[j].find(i) == string::npos;
    }
    if (good) return printf("%c\n", i) & 0;
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = 'a'; j <= 'z'; j++) {
      char buf[3];
      buf[0] = i;
      buf[1] = j;
      bool good = true;
      for (int k = 0, _k = N; k < _k; ++k) {
        good &= vec[k].find(buf) == string::npos;
      }
      if (good) return printf("%s\n", buf) & 0;
    }
  }
  return 0;
}
