n = gets.chomp.to_i
x = []
y = []
length = 0
res = 1
result = 0

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

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

for i in 1..n-1
    res *= i
end

puts (length * (res) * 2) / (res * n)