# coding: utf-8


def main():
    n = int(input())
    v = list(map(int, input().split()))

    ans = []

    while(v.count(0) != len(v)):
        ans.append(max(v))
        index = (max(enumerate(v), key=lambda x: x[1])[0])

        v[index] = 0
        if index > 0:
            v[index-1] = 0
        if index < len(v)-1:
            v[index+1] = 0

    print(sum(ans))


if __name__ == '__main__':
    main()