n = gets.to_i
x=Array.new(n)
y=Array.new(n)

for i in 0..n-1
  a,b=gets.split(' ').map(&:to_i)
  x[i]=a
  y[i]=b

end
length = Array.new((n-1)*n/2)
count = 0
for i in 0..n-2
  for j in i+1..n-1
    length[count]=Math.sqrt((x[i]-x[j])**2+(y[i]-y[j])**2)
    count+=1
  end
end

ans = length.inject(:+)*2*(n-1)/n/(n-1)

puts ans

