If \[ \Delta_r = \begin{vmatrix} 1 & n & n \\ 2r & n^2+n+1 & n^2+n \\ 2r-1 & n^2 & n^2+n+1 \end{vmatrix} \] and \( \sum_{r=1}^{n} \Delta_r = 56 \), then \( n \) is