TUOI PASCAL

var	cha,con,x: longint;
begin
	assign(input,’tuoi.inp’);reset(input);
	assign(input,’tuoi.out’);rewrite(output);
	read(cha,con);
	x:=0;
	while cha+x>2*(con+x) do 
		x:=x+1;
	writeln(x);
end.