def dist(sx, sy, ex, ey)
    dx = ex-sx
    dy = ey-sy
    return (dx*dx + dy*dy)**0.5
end

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

sum = 0
a.permutation(n) {|b|
    for i in 0...n-1
        s = b[i]
        t = b[i+1]
        sum += dist(x[s], y[s], x[t], y[t])
    end
}

nn = 1
for i in 1..n
    nn *= i
end

puts sum / nn

