N = gets.to_i
points = []
N.times do 
    points << gets.split.map(&:to_i)
end

sum = 0.0
for i in 0..N-2
    for j in i+1..N-1
        sum += Math.sqrt((points[i][0] - points[j][0]) ** 2 + (points[i][1] - points[j][1]) ** 2)
    end
end

p sum * 2 * (1..N-1).to_a.inject(:*) / (1..N).to_a.inject(:*)