program snaga;
uses crt;
var a,b,i,br,z,j,k,p:longint;

begin
read(a,b);
for i:=a to b do begin
      p:=i;
       br:=0;
     repeat
     for j:=1 to p do begin
         if (p mod j<>0) then begin
            if j<p then begin k:=j;
              br:=br+1;
            end;
            end;

            end;
        p:=k;

       until k=2;
end;
write(br);
end.