program dem_ngto;
uses crt;
var a,b,i,d:integer;
function ktnt(k:integer):boolean;
var l:integer;
begin
ktnt:=false;
if k<2 then exit;
for l:=2 to trunc(sqrt(k)) do
if k mod l=0 then exit;
ktnt:=true;
end;
BEGIN
clrscr;
write('a= '); readln(a);
write('b= '); readln(b);
d:=0;
for i:=a to b do
if ktnt(i) then d:=d+1;
write(d);
readln;
end.