(defn split-space [s]
  (map #(Long/parseLong %) (.split s " ")))

(defn read-n [n]
  (for [i (range n)] (split-space (read-line))))

(defn square [x]
  (* x x))

(defn distance [d [xi yi] xs]
  (if (zero? (count xs))
    d
    (recur (reduce (fn [acc [xj yj]]
                     (+ acc (Math/sqrt (+ (square (- xi xj))
                                          (square (- yi yj)))))) d xs)
           (first xs)
           (rest xs))))

(defn average-length []
  (->>
   (Long/parseLong (read-line))
   ((fn [n]
      (->>
       (read-n n)
       ((fn [xs] (distance 0 (first xs) (rest xs))))
       ((fn [s] (/ (* 2 s) (* n)))))))))

(println (average-length))