program c;

procedure check(a, b, c: longint; k: byte);
var
	pretykac: longint;
	i: longint;
	sumx: longint;
	ok: boolean;
	amin: longint;
	timeout: longint;
begin
	if c=0 then if a=b then begin
		writeln('0');
		exit;
	end else begin
		writeln('FOREVER');
		exit;
	end;
	pretykac := 1 shl k;
	amin := pretykac;
	ok := false;
	timeout := 200000;
	for i:=0 to pretykac do begin
		sumx := b + pretykac*i - a;
		if (sumx>=0) and ((sumx mod c)=0) then begin
			writeln(sumx div c);
			{
				writeln('sumx: ', sumx);
				writeln('pretykac: ', pretykac);
				writeln('i: ', i);
				writeln('c: ', c);
			}
			ok := true;
		end;
		if ok then break;
		dec(timeout);
		if timeout<=0 then break;
	end;
	if not(ok) then writeln('FOREVER');
end;


var
	a, b, c, k: longint;

begin
while true do begin
	readln(a, b, c, k);
	if (a=0) and (b=0) and (c=0) and (k=0) then break;
	check(a, b, c, k);
end;
end.