program Project1;
var f:text;i,n,k:integer;a,b,x,y:array [1..100] of integer;
begin
assign(f,'Input.dat');reset(f);
readln(f,n);
for i:=1 to n do
readln(f,x[i],y[i]);
close(f);
for i:=1 to n-1 do begin
a[i]:=x[i+1]-x[i];b[i]:=y[i+1]-y[i];end;
k:=0;assign(f,'output.sol');rewrite(f);
for i:=1 to n-1 do
if a[i]*b[i+1]-a[i+1]*b[i]>0 then
k:=k+1;
writeln(f,k);close(f);
end.