Program hotrotinhoc;

var max,i,n: integer;

x,s: string;

d: array['A'..'z'] of integer;

l: char;

begin

readln(x);

for i:=1 to length(x) do

inc(d[x[i]]);

for i:=1 to length(x) do if pos(x[i],x)=i then s:=s+x[i];

for l:='A' to 'z' do if d[l]<>0 then

if d[l]>max then max:=d[l];

for l:='A' to 'z' do if d[l]=max then begin write(l); break; end;

readln;

end.