from itertools import accumulate

N = int(input())
A = list(map(int, input().split()))

ae = list(accumulate([0] + A[0::2]))
ao = list(accumulate([0] + A[1::2]))

a = max(ae[i] - ao[i] for i in range(N+1))
ans = ao[N] - ae[N] + 2 * a
print(ans)