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Extrapolating backwards for a two by two matrix we get the correct formula on the proviso we definfe $J_1\equiv-1$. This makes some sense, as for any $J_nJ_n=I$ and $J_nJ_nA=A$.  We can further extrapolate to the inverse of a 1x1 matrix $A=A_11$, $A=A_{11}$,  taking the $R_11$ $R_{11}$  element to be the zero matrix, the determinant of this matrix is $1$ and the reciprocal of the determinant of $A$ is then just the reciprocal of $A_11$, $A_{11}$,  which again is the inverse of the 1x1 matrix.