#include <bits/stdc++.h>
#define FOR(i,j,k) for(int i=j;i<=k;++i)
#define NFOR(i,j,k) for(int i=j;i>=k;--i)
#define mkp make_pair
#define fst first
#define sec second
#define inl inline
#define pb push_back
using namespace std;
typedef long long ll;
typedef double db;
typedef unsigned int ui;
typedef pair< int,int > pii;
inline int read()
{
	int s=0,w=1; char ch=getchar();
	while(ch<'0'||ch>'9') {if(ch=='-') w=-1; ch=getchar();}
	while(ch>='0'&&ch<='9') {s=(s<<1)+(s<<3)+ch-'0',ch=getchar();}
	return s*w;
}
void file()
{
	freopen(".in","r",stdin);
	freopen(".out","w",stdout);
}
void teltim(int x)
{
	clock_t c1=0;
	if(x) c1=clock();
	else cerr<<endl<<clock()-c1<<"ms"<<endl;
}
const int N=5e5+5,B=30;
const db pi=acos(-1);
typedef pair < db,db > pdd;

int n;
db ans,tmp[N],s,x[N],y[N];

int main()
{
	//file();
	n=read();
	x[0]=1;
	y[0]=0;
	FOR(i,1,n) x[i]=read(),y[i]=read();
	for(int g=0;g<B;++g)
	{
		FOR(i,1,n) tmp[i]=x[i];
		sort(tmp+1,tmp+n+1);
		s+=abs(x[0]);
		FOR(i,1,n) ans+=(2*i-n-2)*tmp[i];
		FOR(i,0,n)
		{
			db xx=x[i]*cos(2*pi/B)-y[i]*sin(2*pi/B);
			db yy=x[i]*sin(2*pi/B)+y[i]*cos(2*pi/B);
			x[i]=xx;
			y[i]=yy;
		}
	}
    printf("%.10lf",ans/s);
	return 0;
}
/*
3
-1 2
2 2
-1 -2
*/
//12.0000000000