data = [1, 2, 1,0,1] n = 5 def backtrack(num, a,b):     if num==n:         # print(a)         lab[0]=lab[0]+a         lab[1]=lab[1]+1     else:         if data[num]==2:             b1=b+1             a1=a+b1             backtrack(num+1,a1,b1)             b2=0             backtrack(num+1,a,b2)         elif data[num]==1:             b=b+1             a=a+b             backtrack(num+1,a,b)         else:             b=0             backtrack(num+1,a,b) lab = [0, 0] backtrack(0,0,0) print(lab[0]//lab[1])