N = int(input())
delicious = []
delicious.extend(list(map(int,input().split())))

sum  = 0

while max(delicious) != 0:
    a = max(delicious)
    b = delicious.index(a)
    sum += a
    delicious[b] = 0
    if b != 0 :
        delicious[b - 1] = 0

    if len(delicious) == b + 1 :
        continue
    else:
        delicious[b + 1] = 0
 
print(sum)