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

sum  = 0

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

    if len(delicious)  < b  + 1 :
        continue
    else:
        del delicious[b]
 
print(sum)