var
key:boolean;
m,n,k3:integer;
k:double;
begin
Readln(n);
m:=1;
key:=true;
while key do
begin
k3:=n-m;
if k3 mod 3 = 0 then
key:=false;
m:=m+1;
end;
k:=k3/3;
writeln(k);
end.
var
key:boolean;
m,n,k3:integer;
k:double;
begin
Readln(n);
m:=1;
key:=true;
while key do
begin
k3:=n-m;
if k3 mod 3 = 0 then
key:=false;
m:=m+1;
end;
k:=k3/3;
writeln(k);
end.