#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=200010;
struct P{
    ll x,y;
};
P a[N],s[N];
ll M(P u,P v)
{
    ll tmp=1ll*u.x*v.y-1ll*v.x*u.y;
    return tmp;
}
int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%lld %lld",&a[i].x,&a[i].y);
        s[i].x=1ll*s[i-1].x+1ll*a[i].x;
        s[i].y=1ll*s[i-1].y+1ll*a[i].y;
    }
    ll pre_ans=0ll;
    ll pre_s=0ll;
    for(int i=1;i<=n;i++)
    {
        pre_s=pre_s*1ll+1ll*M(a[i],a[1+(i)%n]);
    }
    for(int i=1;i<=n;i++)
    {
        P t1,t2;
        t1.x=1ll*(i-1)*a[i].x-1ll*s[i-1].x;
        t1.y=1ll*(i-1)*a[i].y-1ll*s[i-1].y;
        t2.x=1ll*s[n].x-1ll*s[i].x-(n-i)*1ll*a[i].x;
        t2.y=1ll*s[n].y-1ll*s[i].y-(n-i)*1ll*a[i].y;
        //printf("%lld %lld\n",t1.x,t1.y);
        //printf("%lld %lld\n",t2.x,t2.y);
        //printf("%lld\n",M(t1,t2));
        pre_ans=pre_s*1ll+1ll*M(t1,t2);
    }
    //printf("%lld %lld\n",pre_s,pre_ans);
    double ans=(pre_ans*1.00)/(1.00*pre_s);
    printf("%.8f\n",ans);
    return 0;
}