Let there be a regular Heptagon inscribed in a circle of radius $357$ units. Find the sum of the square of distances between all pairs of vertices of the Heptagon.In other words, let $v_1,v_2,v_3 ... v_7$ be the vertices of the Heptagon, and $D_{ij}$ be the distance between vertex $v_i$ and $v_j$. Find $S=\sum_{1\leq i < j \leq 7} D_{ij}^2$