euler 28

gistfile1.py

def approach1(size):
  diagonal = 0
  current = 1
  for i in range(1, size+1):
    current += i
    diagonal += current
    # decrease the sum if it is the first odd turn
    if i%2 != 0:
      diagonal -= 1
    current += i
    diagonal += current

  # remove the last turn
  diagonal -= current
  print diagonal