n = input()

arr = map(int, raw_input().split())

maxes = []

while sum(arr) != 0:
    mx = arr.index(max(arr))
    maxes.append(arr[mx])
    arr[mx] = 0
    if mx - 1 >= 0:
        arr[mx - 1] = 0
    if mx + 1 < len(arr):
        arr[mx + 1] = 0

print sum(maxes)