def selectmax(a1,a2,a3,a4):
    left = a1 + a3
    right = a2 + a4
    edge = a1 + a4
    max1 = max(left,right,edge)
    if max1 == right:
        return 'right'
    else:
        return 'left'


n = int(input())
m = input().split()
v = [0]
sum = []
for num in range(n):
    v.append(int(m[num]))

judge = True
i = 1
num2 = n
sum = 0
while judge:
    judge = False
    if num2 == 1:
        sum += v[i]
    elif num2 == 2:
        sum += max(v[i],v[i+1])
    elif num2 == 3:
        sum += max(v[i]+v[i+2],v[i+1])
    else:
        judge = True
        b1 = v[i]
        b2 = v[i+1]
        b3 = v[i+2]
        b4 = v[i+3]
        message = selectmax(b1,b2,b3,b4)
        if message is 'right':
            sum += b2
            i += 3
            num2 -= 3
        else:
            sum += b1
            i += 2
            num2 -= 2
print(sum)