program xx2010_1;
var i,j,s:longint;prime:boolean;
begin
for i:=2 to 20 do begin
prime:=true;
for j:=2 to i-1 do
if i mod j=0 then begin
prime:=false;
break;
end;
if prime then s:=s+1;
end;
writeln(s);
end.
输出：8