const N=50;
var
a : array [1..N] of boolean;
x,y, sim : integer;
begin
sim:=0;
a[1] := false;
for x:=2 to N do a[x] := true;
for x:= 2 to N div 2 do
for y:= 2 to N div x do
a[x*y] := false;
for x:=1 to N do
if a[x] then sim:=sim+1;
writeln(sim);
end.