Var
  S,R: Array[1..1000] of Char;
  I,J,N,ML,L: Integer;
  T: Boolean;
Begin
{  Assign(Input,'Input.txt');
  Reset(Input);}
  FillChar(S,SizeOf(S),32);
  FillChar(R,SizeOf(R),32);
  N:=0;
  While not EOF do begin
    Inc(N);
    Read(S[N]);
  End;
  For I:=1 to N do
    For L:=ML to N-I+1 do begin
      T:=True;
      For J:=1 to (L div 2) do
        If S[I+J-1]<>S[I+L-J] then begin
          T:=False;
          Break;
        End;
      If T and (L>ML) then begin
        ML:=L;
        For J:=1 to ML do
          R[J]:=S[I+J-1];
      End;
    End;
  For I:=1 to ML do
    Write(R[I]);
End.