output_description: Print the maximum possible sum of the chosen elements.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 ... A_N
attempt: k = int(input())
  l = list(map(int, input().split()))
  odd = 0
  even = 0
  ct = 0
  c = 0
  le = []
  lo = []
  odd_mn = -1000000001
  odd_mn1 = -1000000001
  even_mn = -1000000001
  even_mn = -1000000001
  for i in range(k):
      if i % 2 == 0:
          if l[i] >= 0:
              c += 1
              even += l[i]
          if l[i] > even_mn and l[i] < 0:
              even_mn = l[i]
              ind = i
          le.append(l[i])
      else:
          if l[i] >= 0:
              odd += l[i]
              ct += 1
          if l[i] > odd_mn and l[i] < 0:
              odd_mn = l[i]
              ino = i
          lo.append(l[i])
  if c == 1:
      even += even_mn
  if ct == 1:
      odd += odd_mn
  if c == 0:
      even += even_mn
      even_mn = -1000000001
      for i in range(len(le)):
          if l[i] > even_mn and ind != i // 2:
              even_mn = l[i]
      even += even_mn
  if ct == 0:
      odd += odd_mn
      odd_mn = -1000000001
      for i in range(len(lo)):
          if l[i] > odd_mn and ino != (i // 2 + 1):
              odd_mn = l[i]
      odd += odd_mn
  
  if odd > even:
      print(odd)
  else:
      print(even)
  
problem_description: Statement
  
  Given is an integer sequence A_1, ..., A_N of length N.
  
  We will choose exactly \left\lfloor \frac{N}{2} \right\rfloor elements from
  this sequence so that no two adjacent elements are chosen.
  
  Find the maximum possible sum of the chosen elements.
  
  Here \lfloor x \rfloor denotes the greatest integer not greater than x.
samples: [{"input": "6\n    1 2 3 4 5 6", "output": "12\n    \n\nChoosing 2, 4, and 6 makes the sum 12, which is the maximum possible value.\n\n* * *"}, {"input": "5\n    -1000 -100 -10 0 10", "output": "0\n    \n\nChoosing -10 and 10 makes the sum 0, which is the maximum possible value.\n\n* * *"}, {"input": "10\n    1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000", "output": "5000000000\n    \n\nWatch out for overflow.\n\n* * *"}, {"input": "27\n    18 -28 18 28 -45 90 -45 23 -53 60 28 -74 -71 35 -26 -62 49 -77 57 24 -70 -93 69 -99 59 57 -49", "output": "295"}]