class Node:
    def __init__(self, v):
        self.v = v
        self.c = []

def tree(li):
    n = [Node(i) for i in li]
    for i in range(1, len(li)):
        n[(i-1)//2].c.append(n[i])
    return n[0]

def calc(n, level=0):
    return n.v if level % 2 else 0 + \
        sum(calc(n, level+1) for n in n.c) if n else 0

li = [3, 5, 8, 12, 15, 17, 21]
root = tree(li)
print(calc(root))