#include<bits/stdc++.h>
#define Gc() getchar()
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define eb emplace_back
#define all(x) x.begin(),x.end()
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;
const int N=5e5+5,M=N*4+5,K=1e7+5,mod=1e9+7,Mod=mod-1;const db eps=1e-9;const ll INF=1e18+7;mt19937 rnd(263082);
#define Tp template<typename T>
#define Ts template<typename T,typename... Ar>
namespace Debug{
	Tp void _debug(char* f,T t){cerr<<f<<'='<<t<<endl;}
	Ts void _debug(char* f,T x,Ar... y){while(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
	#ifdef LOCAL
	#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
	#else 
	#define gdb(...) void()
	#endif
}using namespace Debug;
int n;
int X[N],Y[N];
db A[N];
const db pi=acos(-1);
void Solve(){
	scanf("%d",&n);
	for(int i=1;i<=n;i++) scanf("%d%d",&X[i],&Y[i]);
	db tot=0;int cnt=0;
	while(clock()*1.0/CLOCKS_PER_SEC<0.95){
		db ap=R(1000000)*2*pi/1000000.0;
		db x=cos(ap),y=sin(ap);
		for(int i=1;i<=n;i++) A[i]=X[i]*x+Y[i]*y;
		sort(A+1,A+n+1);
		for(int i=1;i<=n;i++) tot+=A[i]*(2*i-n-1);
		cnt++;
	}
	printf("%.9lf\n",tot*pi/cnt/2);
}
int main(){
	int t=1;
	// scanf("%d",&t);
	while(t--) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}