{$APPTYPE CONSOLE}
Var
  S,st:AnsiString;
  i,N,d,MaxLen:Integer;
begin
  Readln(S);
  N:=length(S); MaxLen:=0;
  for i:= 1 to N do begin
    d:=0;
   while (i>=d)And((N-i)>=d)And(S[i-d]=S[i+d])do Inc(d);
    if d>MaxLen then begin
      MaxLen:=d;
      st:=Copy(S,i-d+1,d*2-1);
    end;
  end;
  writeln(st);
end.

try this string: aooa

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