n = int(input())
sushi_plates = list(map(int, input().split()))
sushi_plates.reverse()

if n == 1:
    print(sushi_plates.pop())
    exit()

total = 0
index = 0
while index < n - 1:
    first = sushi_plates[index]
    second = sushi_plates[index + 1]

    if first > second:
        total += first
        index += 2
    else:
        total += second
        index += 3

print(total)