#!/usr/bin/env python3
P = [[float(x) for x in input().split()] for i in range(int(input()))]
A,L = 0,0
for i in range(len(P)):
    A -= (P[i-1][1]+P[i][1])*(P[i-1][0]-P[i][0])
    L += (P[i][0]-P[i-1][0])**2+(P[i][1]-P[i-1][1])**2
A /= 2
L -= 1
print(3**.5/4+(3*(A/(1-L)) if L<1 else 0))