C4 = G3 + P3G2 + P3P2G1 + P3P2P1Go +P3P2P1PoCo
Gi=XiYi , Pi=Xi+Yi
Ci+1 = Gi + Pi(Ci)
At level1: G3 =X3Y3, P3=X3+Y3, G2 =X2Y2, P2 =X2+Y2 ,G1 =X1Y1, P1 =x1+Y1,G0 =XoYo,Po=Xo+Y0 (AND OR GATES is used)
At level2: P3G2 , P3P2G1 , P3P2P1Go ,P3P2P1PoCo (AND GATES is used)
At level3: C1 C2 C3 C4
At level 4: Sum you will get.
Average Gate delay is d and K level mentioned Kd.