n=gets.to_i
city_x=[]
city_y=[]

n.times.each do |i|
    city_x[i],city_y[i]=gets.split.map(&:to_i)
end

all_number=[*1..n]
all_pattern=all_number.permutation.to_a
#p city_x
#p city_y

result=[]

for i in 0..all_pattern.length-1
    v=0
    i=all_pattern[i]
    for j in 0..n-2
        x=(city_x[i[j]-1]-city_x[i[j+1]-1])**2
        y=(city_y[i[j]-1]-city_y[i[j+1]-1])**2
        #p x
        #p y
        v+=(x+y)**(0.5)
        #p v
    end
    
    result<<v
end
#p result
p result.inject(:+) / result.length
