program z1;
var x,m,n: integer;
f: boolean;
begin
readln(x);
f: =true;
m: =x mod 10; x: = x div 10;
while x< > 0 do begin
n: =x mod 10;
x: =x div 10;
if n> m then begin
f: =false;
break
end;
m: =n
end;
if f then writeln('yes')
else writeln('no');
readln
end.
