/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
- insert(x),erase(x)
- find_by_order(k): return iterator to the k-th smallest element
- order_of_key(x): the number of elements that are strictly smaller
*/

#include<bits/stdc++.h>
using namespace std;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
uniform_real_distribution<> pp(0.0,1.0);
#define int long long
#define ld long double
#define pii pair<int,int>
#define piii pair<int,pii>
#define mpp make_pair
#define fi first
#define se second
const int inf=1e18;
const int mod=998244353;
const int maxn=3005;
const int bl=650;
const int maxs=655;
const int maxm=200005;
const int maxq=1000005;
const int maxl=25;
const int maxa=1000000;
const int root=3;
int power(int a,int n){
    int res=1;
    while(n){
        if(n&1) res=res*a%mod;
        a=a*a%mod;n>>=1;
    }
    return res;
}
const int iroot=power(3,mod-2);
const int base=131;

double PI = acos(-1.0);

struct vec{
    int x=0,y=0;
    double d;
    vec(int x=0,int y=0):x(x),y(y){
        d=atan2(y,x);
    }
    bool operator<(vec a){
        return d<a.d;
    }
}P[2*maxn];

int n,X[maxn],Y[maxn];
int total=0;

void solve(){
    cin >> n;
    for(int i=1;i<=n;i++) cin >> X[i] >> Y[i];
    int num=0;
    for(int t=1;t<=n;t++){
        for(int i=1;i<t;i++) P[i]=vec(X[i]-X[t],Y[i]-Y[t]);
        for(int i=t+1;i<=n;i++) P[i-1]=vec(X[i]-X[t],Y[i]-Y[t]);
        sort(P+1,P+n);
        int B=0;
        for(int i=1;i<n;i++){
            P[i+n-1]=P[i];
            P[i+n-1].d+=2*PI;
        }
        for(int i=1,j=1;i<n;i++){
            while(P[i].d+PI>P[j].d) B++,j++;
            num+=(B-1)*(n-1-B);B--;
        }
    }
    num/=2;
    int k=num-(n*(n-1)*(n-2)*(n-3)/12);
    num=(num-3*k)/2;
    int cnt=n*(n-1)*(n-2)/6;
    total+=num*2+k;
    double ans=3+(double)total/cnt;
    cout << setprecision(10) << fixed << ans << '\n';
}

signed main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    int test=1;//cin >> test;
    while(test--) solve();
}