N = gets.chomp.to_i
a = Array.new(N)
b = Array.new(N)
for i in 0...N do
    str = gets.split
    a[i],b[i] = str[0].to_i, str[1].to_i
end
d = 0
e = 0
for i in 0...N-1 do
    for j in i+1...N do
        aa = (a[i]-a[j])**2
        bb = (b[i]-b[j])**2
        d += Math.sqrt(aa+bb)
        e += 1
    end
end
d /= e*1.0
answer = d*(N-1)
puts answer