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: Nin = int(input())
  guki = Nin % 2
  N = Nin // 2
  A = list(map(int, input().split()))
  summost = sum(A[1::2])
  if guki == 0:
      for a in range(0, N):
          if summost < sum(A[: 2 * a : 2] + A[2 * a + 1 :: 2]):
              summost = sum(A[: 2 * a : 2] + A[2 * a + 1 :: 2])
  else:
      for a in range(0, N):
          for b in range(a, N):
              if summost < sum(
                  A[: 2 * a : 2] + A[2 * a + 1 : 2 * b + 1 : 2] + A[2 * b + 2 :: 2]
              ):
                  summost = sum(
                      A[: 2 * a : 2] + A[2 * a + 1 : 2 * b + 1 : 2] + A[2 * b + 2 :: 2]
                  )
  print(summost)
  
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"}]