N = int(input())
V = tuple(map(int, input().split()))
s = 0
e = False

if N > 2:
    for i in range(N-1, 1, -1):
        if e:
            if V[i-2] > V[i-1]:
                e = True
            else:
                e = False
        else:
            if V[i] + V[i-2] > V[i-1]:
                s += V[i]
                e = True
            else:
                e = False
    else:
        if e:
            s += V[0]
        else:
            s += max(V[0], V[1])
    print(s)
else:
    print(max(V))