n = gets.to_i
x = []
y = []
n.times do
  a,b = gets.split.map(&:to_i)
  x << a
  y << b
end



if n == 2
  puts Math.sqrt((((x[0]-x[1])**2)+((y[0]-y[1])**2)).to_f)
else

bikkuri = 1
for i in 1..n
bikkuri *= i
end

z = ((2*(n-1)*(n-2)).to_f)/bikkuri
sum = 0

for i in 0..(n-1)
  for j in (i+1)..(n-1)
    root = Math.sqrt((((x[i]-x[j])**2) + ((y[i]-y[j])**2)).to_f)
    sum += root
  end
end

puts sum*z
end