# -*- coding:utf8 -*-
# Reads a file containing densities per link and computes the flow density for this delta.
# Date : December 2013
# Author : tvd
import sys

contents = 0.0
i = 0
total_nodes = 0

for line in sys.stdin:
	if i == 0:
		total_nodes = int(line.strip())
	else:
		contents += float(line.strip())

	i += 1

print "%1.15f" % (contents/((total_nodes*(total_nodes - 1))/2))