begin
var s := '''
ABCD
CABD
ACDB
DACB
BCDA
ACBD
ADCB
CDAB
DABC
BCAD
CADB
CDBA
CBAD
ABDC
ADBC
BDCA
DCBA
BACD
BADC
BDAC
CBDA
DBCA
DCAB
''';
var perms := s.ToLines;
Print(('ABCD'.Permutations.ToHashSet - perms.ToHashSet).First);
end.