Uses crt;
const n=8;
var a:integer;
b:array[1..n] of integer;
i,k,t:integer;
begin
readln(a);
k:=1;
repeat
for i:=k to k do
b[i]:=a mod 10;
a:=a div 10;
inc(k);
until a=0;
t:=0;
for i:=k-1 downto 2 do
if (b[i]>b[i-1]) then inc(t);
if t=0 then writeln('yes') else writeln('no');
end.