def longest_slide_down(pyramid):
    paths = [0]
    for step in pyramid:
        print(paths)
        paths = [paths[0]+step[0]] + list([max(da+a,db+b) for da,a,db,b in zip(step[:-1],paths[:-1],step[1:],paths[1:])]) + [paths[-1]+step[-1]]
    print(paths)

    return max(paths)

print(longest_slide_down([[3],[7,4],[2,4,6],[8,5,9,3]]))