A cyclic quadrilateral ABCD is given. Point K1,K2 lie on the segment AB, points L1,L2 on the segment BC, points M1,M2 on the segment CD and points N1,N2 on the segment DA. Moreover, points K1,K2,L1,L2,M1,M2,N1,N2 lie on a circle ω in that order. Denote by a,b,c,d the lengths of the arcs N2K1,K2L1,L2M1,M2N1 of the circle ω not containing points K2,L2,M2,N2, respectively. Prove that
\begin{align*}
a+c=b+d.
\end{align*} geometryarcscyclic quadrilateral