A了这道题

#include<bits/stdc++.h>
using namespace std;
#define cs const
#define re register
#define pb push_back
#define pii pair<int,int>
#define ll long long
#define fi first
#define se second
#define bg begin
cs int RLEN=1<<20|1;
inline char gc(){
    static char ibuf[RLEN],*ib,*ob;
    (ib==ob)&&(ob=(ib=ibuf)+fread(ibuf,1,RLEN,stdin));
    return (ib==ob)?EOF:*ib++;
}
inline int read(){
    char ch=gc();
    int res=0;bool f=1;
    while(!isdigit(ch))f^=ch=='-',ch=gc();
    while(isdigit(ch))res=(res+(res<<2)<<1)+(ch^48),ch=gc();
    return f?res:-res;
}
inline ll readll(){
    char ch=gc();
    ll res=0;bool f=1;
    while(!isdigit(ch))f^=ch=='-',ch=gc();
    while(isdigit(ch))res=(res+(res<<2)<<1)+(ch^48),ch=gc();
    return f?res:-res;
}
inline int readstring(char *s){
	int top=0;char ch=gc();
	while(isspace(ch))ch=gc();
	while(!isspace(ch)&&ch!=EOF)s[++top]=ch,ch=gc();
	return top;
}
template<typename tp>inline void chemx(tp &a,tp b){a<b?a=b:0;}
template<typename tp>inline void chemn(tp &a,tp b){a>b?a=b:0;}
cs double eps=1e-9;
inline double reps(){
	return ((1.0*rand()/RAND_MAX)-0.5)*eps;
}
cs int N=4005;
struct pt{
	double x,y,z;
	void shake(){x+=reps(),y+=reps(),z+=reps();}
	pt(double _x=0,double _y=0,double _z=0):x(_x),y(_y),z(_z){}
	friend inline pt operator +(cs pt&a,cs pt&b){
		return pt(a.x+b.x,a.y+b.y,a.z+b.z);
	}
	friend inline pt operator -(cs pt&a,cs pt&b){
		return pt(a.x-b.x,a.y-b.y,a.z-b.z);
	}
	friend inline pt operator *(cs pt &a,cs pt &b){
		return pt(a.y*b.z-a.z*b.y,a.z*b.x-a.x*b.z,a.x*b.y-a.y*b.x);
	}
	friend inline double operator ^(pt a,pt b){
		return a.x*b.x+a.y*b.y+a.z*b.z;
	}
	inline double len(){return sqrt(x*x+y*y+z*z);}
}p[N];
struct flat{
	int v[3];
	flat(){}
	flat(int a,int b,int c){v[0]=a,v[1]=b,v[2]=c;}
	inline pt calc()cs{return (p[v[1]]-p[v[0]])*(p[v[2]]-p[v[0]]);}
	inline double S()cs{return calc().len()*0.5;}
}f[N],tp[N];
int cnt,n,vis[N][N];
inline bool See(cs flat &a,cs pt &b){
	return ((b-p[a.v[0]])^(a.calc()))>0;
}
int main(){
	#ifdef Stargazer
	freopen("lx.in","r",stdin);
	#endif
	scanf("%d",&n);
	for(int i=1;i<=n;i++)scanf("%lf%lf%lf",&p[i].x,&p[i].y,&p[i].z),p[i].shake();
	f[1].v[0]=1,f[1].v[1]=2,f[1].v[2]=3;
	f[2].v[0]=3,f[2].v[1]=2,f[2].v[2]=1;
	cnt=2;
	for(int i=4;i<=n;i++){
		int c=0;
		for(int j=1;j<=cnt;j++){
			int t=See(f[j],p[i]);
			if(!t)tp[++c]=f[j];
			for(int k=0;k<3;k++)vis[f[j].v[k]][f[j].v[(k+1)%3]]=t;
		}
		for(int j=1;j<=cnt;j++){
			for(int k=0;k<3;k++){
				int x=f[j].v[k],y=f[j].v[(k+1)%3];
				if(vis[x][y]&&!vis[y][x])tp[++c]=flat(x,y,i); 
			}
		}
		for(int j=1;j<=c;j++)f[j]=tp[j];
		cnt=c;
	}double ans=0;
	for(int i=1;i<=cnt;i++)ans+=f[i].S();
	printf("%.3lf",ans);return 0;
}