lines = []
while line = gets
    lines << line.chomp.split(' ').map(&:to_i)
end

N = lines[0][0]
$x = []
$y = []
for i in 1..N do
  $x.push(lines[i][0])
  $y.push(lines[i][1])
end

def length(i,j)
  return (($x[i-1] - $x[j-1]) ** 2 + ($y[i-1] - $y[j-1]) ** 2) ** 0.5
end

sum_length = 0
for i in 1..N-1 do
  for j in i+1..N do
    sum_length += length(i,j)
  end
end

print 2 * sum_length / N