def osusi(N, V):
    if N == 1:
        return V[0]
    eat1 = [[0], [1]]
    while True:
        eat2 = []
        for i in range(len(eat1)):
            if eat1[i][-1] + 2 <= N - 1:
                app1 = eat1[i] + [eat1[i][-1] + 2]
                eat2.append(app1)
            if eat1[i][-1] + 3 <= N - 1:
                app2 = eat1[i] + [eat1[i][-1] + 3]
                eat2.append(app2)
        eat1 = [] + eat2
        if eat1[-1][-1] + 2 > N - 1:
            break
    V_sum = [0 for x in range(len(eat1))]
    for j in range(len(eat1)):
        for k in range(len(eat1[j])):
            l = eat1[j][k]
            V_sum[j] += V[l]
    result = max(V_sum)
    return result

if __name__ == '__main__':
    N = int(input())
    V_li = list(map(int, input().split()))
    print(osusi(N, V_li))