var
a:array[1..1000] of string;
S, pol:string;
N, i, j, dlina:longint;
function Sum(i, j:integer):string;
var k:string;
var i1:integer;
begin
k:='';
for i1:= i to j do
k:=k+a[i1];
Sum:=k;
end;
function Sum1(i, j:integer):string;
var k:string;
var i1:integer;
begin
k:='';
for i1:= j downto i do
k:=k+a[i1];
Sum1:=k;
end;
begin
readln(S);
N:=length(S);
for i:= 1 to N do
a[i]:=copy(lowercase(S), i, 1);
dlina:=0;
for i:= 1 to N-1 do
begin
for j:= i+1 to N do
if (Sum(i, j)=Sum1(i, j)) and (length(sum(i, j))>dlina) then
begin
pol:=Sum(i, j);
dlina:= length(sum(i, j));
end;
end;
writeln(pol);
end.