CAU3

program CAU3;
var i, j, n, a, b, c:integer;

Begin
  readln(a, b);
  c:=0;
  for n:=a to b do
  begin
    j:=1;
    for i:=2 to n-1 do
    begin
      if n mod i = 0 then j:=0;
    end;
    if (j = 1) and (n > 1) then c:=c+1;
  end;
  writeln(c);
  readln;
End.