import strutils, sequtils, math, algorithm
var
  N, V = stdin.readLine.split.map parseInt
  cnt = 0
  flg: bool

proc check =
  var best = 0
  if V.len <= 4:
    for i in 0 ..< 2 ^ V.len:
      var
        pass: bool
        cnt_s = 0
      for j in 0 ..< V.len:
        if pass:
          pass = false
        elif i.toBin(V.len)[j] == '1':
          cnt_s += V[j]
          pass = true
      best = max(best, cnt_s)
    echo best + cnt
    quit()

proc eat =
  cnt += V[0]
  V.delete 0, 1
  flg = false

while true:
  check()
  if V[0] >= V[1]:
    eat()
  else:
    case V[0] + V[2] - (V[1] + V[3])
    of 0:
      if flg:
        eat()
      else:
        V.reverse
        flg = true
    of 1 .. int.high:
      eat()
    else:
      V.delete 0
      flg = false