Using the following Seifert surface of the cinquefoil knot
I get the following Seifert matrix (of linking numbers): $ S = \begin{pmatrix}- 1 &1 &0 &0 \\ 0 &-1 &1& 0 \\ 0& 0 &-1& 1\\ 0 &0 &0& -1\end{pmatrix}$
I compute the matrix of the corresponding bilinear form $ I = S^T - S = \begin{pmatrix} 0 & -1 & 0 & 0 \\ 1 & 0 & -1 & 0 \\ 0 & 1 & 0 & -1 \\ 0 & 0 & 1 & 0 \end{pmatrix}$
and a corresponding symplectic basis $e_1 = \begin{pmatrix}0 \\0 \\0 \\1 \end{pmatrix}$ $f_1 = \begin{pmatrix} 0 \\1 \\1 \\0\end{pmatrix}$
$f_2 = \begin{pmatrix}1 \\0 \\0 \\0\end{pmatrix}$ $e_2 = \begin{pmatrix}1 \\1 \\1 \\1\end{pmatrix}$
And I get $\mathrm{Arf}(K) = e_1^T S e_1 f_1^T S f_1 + e_2^T S e_2 f_2^T S f_2 = 2 \equiv_2 0 \neq 1$
Where is my mistake? Thanks for your help.