Let $a_1,a_2,a_3,a_4,a_5,$ and $a_6$ be the degree measures of the six angles of a convex hexagon. Suppose that $a_1<a_2<a_3<a_4<a_5<a_6$ and these angles form an arithmetic sequence. If the smallest angle is $90^{\circ }$, find the measure of the largest angle.