N = gets.to_i
A=[]
N.times{ |x|
    A.push(gets.split(' ').map(&:to_i))
}

def jfunc a, b, r
    b.each{|x|
        c = a.dup
        d = b.dup
        c.push(x)
        d.delete(x)
        if d ==[]
            r.push(c)
        else
            jfunc(c,d,r)
        end
    }
end

K = [*(0...N)]

R = []
jfunc([],K,R)

def dist n, m
    Math.sqrt(((n[0] - m[0]) ** 2) + ((n[1] - m[1]) ** 2))
end

result = 0
R.each{ |x|
    (x.length-1).times{|j|
        result = result + dist(A[x[j]], A[x[j+1]])
    }
}

puts result / R.length