output_description: Print the average length of the paths. Your output will be judges as correct
  when the absolute difference from the judge's output is at most 10^{-6}.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      x_1 y_1
      :
      x_N y_N
attempt: n = int(input())
  a = [list(map(int, input().split())) for i in range(n)]
  x = 0
  y = 1
  for i in a:
      for j in a:
          x += pow((i[0] - j[0]) ** 2 + (i[1] - j[1]) ** 2, 0.5)
  for k in range(1, n + 1):
      y *= k
  print(x / y)
  
problem_description: Statement
  
  There are N towns in a coordinate plane. Town i is located at coordinates
  (x_i, y_i). The distance between Town i and Town j is \sqrt{\left(x_i-
  x_j\right)^2+\left(y_i-y_j\right)^2}.
  
  There are N! possible paths to visit all of these towns once. Let the length
  of a path be the distance covered when we start at the first town in the path,
  visit the second, third, \dots, towns, and arrive at the last town (assume
  that we travel in a straight line from a town to another). Compute the average
  length of these N! paths.
samples: [{"input": "3\n    0 0\n    1 0\n    0 1", "output": "2.2761423749\n    \n\nThere are six paths to visit the towns: 1 \u2192 2 \u2192 3, 1 \u2192 3 \u2192 2, 2 \u2192 1 \u2192 3, 2 \u2192 3\n\u2192 1, 3 \u2192 1 \u2192 2, and 3 \u2192 2 \u2192 1.\n\nThe length of the path 1 \u2192 2 \u2192 3 is\n\\sqrt{\\left(0-1\\right)^2+\\left(0-0\\right)^2} +\n\\sqrt{\\left(1-0\\right)^2+\\left(0-1\\right)^2} = 1+\\sqrt{2}.\n\nBy calculating the lengths of the other paths in this way, we see that the\naverage length of all routes is:\n\n\\frac{\\left(1+\\sqrt{2}\\right)+\\left(1+\\sqrt{2}\\right)+\\left(2\\right)+\\left(1+\\sqrt{2}\\right)+\\left(2\\right)+\\left(1+\\sqrt{2}\\right)}{6}\n= 2.276142...\n\n* * *"}, {"input": "2\n    -879 981\n    -866 890", "output": "91.9238815543\n    \n\nThere are two paths to visit the towns: 1 \u2192 2 and 2 \u2192 1. These paths have the\nsame length.\n\n* * *"}, {"input": "8\n    -406 10\n    512 859\n    494 362\n    -955 -475\n    128 553\n    -986 -885\n    763 77\n    449 310", "output": "7641.9817824387"}]