Find it

var M=13;
function func(X,N)
{
	if(((X-1)%M)==0){
		return (func(((X-1)/M)*(M-2),N-1));
	} else if(N>0){
		return (-1);
	} else {
		return (X);
	}
}
function test()
{
	var t=document.myform.myinput.value;
	t>>=30;
	t>>=17; // answer < 2^47
	if (t == 0)
	{
		t=func(document.myform.myinput.value,M);
	}
	if(t>0)
		alert("Right! Just submit it below.");
	else
		alert("Wrong");
}