#coding:utf-8

def main():
    N = int(input())
    dishes =  list(input().split(' '))
    pick_list = pick(N,dishes)
    print(sum(pick_list))

def pick(N,dishes):
    pick_list = []
    i = 0
    while i <= N-1:
        if i == N-1:
            pick_list.append(int(dishes[i]))
            i += 1
        elif dishes[i] >= dishes[i+1]:
            pick_list.append(int(dishes[i]))
            i += 2
        else:
            pick_list.append(int(dishes[i+1]))
            i += 3
    return pick_list
    
if __name__ == '__main__':
    main()