n = int(raw_input())
l = map(int, raw_input().split())
if n == 1:
    print l[0]
else:
    ll = [l[0],max(l[0],l[1])]
    for i in range(2,n):
        ll.append(max(ll[i-1],ll[i-2]+l[i]))
    print ll[-1]