text: import math
  n = int(input())
  
  rl = []
  for i in range(1, n+1):
      rl.append((int(input())**2))
  
  rl.sort(reverse=True)
  red_circle = sum(rl[::2])
  white_circle = sum(rl[1::2])
  print((red_circle-white_circle)*math.pi)
  