import sys

def check(a,b,x):
	if x < a or x > b:
		sys.exit()

def choice(list,i):

	if len(list) <= i:
		return 0

	pick = list[i] + choice(list,i+2)
	npick = choice(list,i+1)

	if pick > npick:
		return pick
	else:
		return npick

N = int(input())
check(1,1000,N)

list = input().split(' ')

for i in range(0,len(list)):
	list[i] = int(list[i])
	check(1,100,list[i])

print(choice(list,0))