import sys
import numpy as np

lines = []
for l in sys.stdin:
    lines.append(l.rstrip('\r\n'))

N = int(lines[0])
V = np.array([0]+[int(i) for i in lines[0].split(" ")]+[0,0])

V_MAX = np.zeros_like(V)

for i in range(V.shape[0]-4,-1,-1):
    V_MAX[i] = V[i]+max(V_MAX[i+2],V_MAX[i+3])
print(V_MAX[0])