function yi = SplineC2natural(x, y, xi)
	n = length(x);
	for i=1:n
		a(i)=y(i);
		if(i < n)
			h(i)=x(i+1)-x(i);
		endif
	endfor

	A = zeros(n, n);

	for i=2:n-1
		A(i,i-1) = h(i-1);
		A(i,i) = 2*(h(i-1)+h(i));
		A(i,i+1) = h(i);
	endfor

	A(1,1) = 1;
	A(n+1, n) = 1;

	B = zeros(n+1,1);

	for i=2:n-2
		B(i)=3*(a(i+1)-a(i))/h(i) - 3*(a(i) - a(i-1))/h(i-1);
	endfor

	c = A\B;

	length(c)
	length(a)
	length(h)

	for i=1:n-1
		b(i) = (a(i+1)-a(i))/h(i) - h(i)/3*(2*c(i) + c(i+1));
		d(i)=(c(i+1)-c(i))/(3*h(i));
	endfor

	for i=1:n-1
	if (xi >= x(i) && xi <= x(i+1))
	yi =a(i)+b(i)*(xi-x(i))+c(i)*(xi-x(i))^2+d(i)*(xi-x(i))^3;
	endif
	endfor