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| #include<bits/stdc++.h>
#define int long long
#define ull unsigned long long
#define maxn 3005
#define put() putchar('\n')
#define Tp template<typename T>
#define Ts template<typename T,typename... Ar>
using namespace std;
Tp void read(T &x){
int f=1;x=0;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') {x=x*10+c-'0';c=getchar();}
x*=f;
}
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...);}
#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
}using namespace Debug;
#define fi first
#define se second
#define mk make_pair
const int mod=1e9+7;
int power(int x,int y=mod-2) {
int sum=1;
while (y) {
if (y&1) sum=sum*x%mod;
x=x*x%mod;y>>=1;
}
return sum;
}
struct node {
int x,y;
node operator -(const node tmp) const {
return {x-tmp.x,y-tmp.y};
}
}a[maxn],c[maxn];
int ans;
int n;
int cross(node x,node y) {return x.x*y.y-x.y*y.x;}
bool cmp1(node x,node y) {return x.x==y.x?x.y<y.y:x.x<y.x;}
bool cmp2(node x,node y) {return cross(x,y)>0;}
signed main(void){
freopen("1.in","r",stdin);
int i,j;
read(n);
for (i=1;i<=n;i++) read(a[i].x),read(a[i].y);
sort(a+1,a+1+n,cmp1);
for (i=1;i<=n;i++) {
int tot=0;node tmp={0,0};
for (j=i+1;j<=n;j++) c[++tot]=a[j]-a[i];
sort(c+1,c+1+tot,cmp2);
for (j=tot;j;j--) {
ans+=cross(c[j],tmp);
tmp.x+=c[j].x;
tmp.y+=c[j].y;
}
}
if (ans&1) printf("%lld.5\n",ans/2);
else printf("%lld.0\n",ans/2);
return 0;
}
|