program uni;
var
a:string[80];
b:array[1..80]of char;
br,i,j,z:integer;
k:char;
begin
br:=0;
writeln('');
readln(a);
z:=length(a);
for i:=1 to z do begin
b[i]:=a[i];
end;
for i:=z downto 1 do begin
for j:=1 to z do begin
if i<>j then begin
if b[i]<>b[j] then  br:=br+1;
if br=z-1 then k:=b[i]else br:=0;
end;end;end;
writeln(k);
readln;
end.