n = gets.chomp.to_i
i=0
xy = []
while true
    tmp = gets.chomp.split.map(&:to_i)
    xy.push(tmp)
    i += 1
    if n == i
        break
    end
end
xy2 = xy.permutation.to_a
d = Array.new(xy2.size,0)
j = 0
for j in 0..(xy2.size-1)
    temp = 0.0
    k = 0
    for k in 0..(n-2)
        temp += ((xy2[j][k][0] - xy2[j][k+1][0]) ** 2 + (xy2[j][k][1] - xy2[j][k+1][1]) ** 2) ** 0.5
    end
    d[j] += temp
end

sum = d.inject(:+)
ave = sum / d.size
print ave