program MaxPal;
var
	s,p : string;
	i,k,n : integer;
{----------------------------------------------------------}
function IsPal(s:string):boolean; {detects where s is palindrome}
var 
	i,n : integer;
begin
IsPal := true;
n := length(s);
for i := 1 to n div 2 do
	if (s[i] <> s[n-i+1]) then 
		begin
		IsPal := false;
		break;
		end;
end;
{----------------------------------------------------------}
begin
readln(s);
n := length(s);
k := n; {we are hunting for palindrome of k chars}
p := '';
while (k > 0) AND (p = '') do
	begin
	for i := 1 to n-k+1 do {possible start positions for k-palindrome}
		begin
		if IsPal(copy(s,i,k)) then
			begin
			p := copy(s,i,k);
			break;
			end;
		end;
	dec (k) {no k-palindrome, lower our expectations}
	end;
writeln (p);
end.