import numpy as np
n = int(input())
A = list(map(int,input().split()))
L = [a for a in A[::2]]
R = [a for a in A[1::2]]
cumL = np.cumsum(L)
cumR = np.cumsum(R)
sumA = sum(A)
ans = 0
ans = max(cumL[-1], cumR[-1])*2 - sumA
for i in range(n):
    h = cumL[i] + cumR[-1] - cumR[i]
    t = sumA - h
    ans = max(ans, h-t)
print(ans)