import numpy as np
n = int(input())
a = np.array(list(map(int, input().split())) + [0] * (3 * 3 - n % 3))
length = len(a)

free, fix = 0, 0
oxo = [True, False, True]
xxo = [False, False, True]
oxx = [True, False, False]
xox = [False, True, False]

def f(array, pos):
    return np.sum(a[pos:pos + 3] * array)

pos = 0

for i in range(length // 3):
    tmp_fix = max(
        free + f(oxo, pos),
        free + f(xxo, pos),
        fix  + f(xxo, pos),
    )
    free = max(
        fix  + f(xox, pos),
        free + f(oxx, pos),
        free + f(xox, pos),
    )
    fix = tmp_fix
    pos += 3

print(max(fix, free))