1 #include<cstdio> 2 #include<cstdlib> 3 #include<cmath> 4 #include<cstring> 5 #include<algorithm> 6 #include<iostream> 7 #include<vector> 8 #include<map> 9 #include<set>10 #include<queue>11 #include<string>12 #define inf 100000000013 #define maxn 3000+50014 #define maxm 500+10015 #define eps 1e-1016 #define ll long long17 #define pa pair<int,int>18 #define for0(i,n) for(int i=0;i<=(n);i++)19 #define for1(i,n) for(int i=1;i<=(n);i++)20 #define for2(i,x,y) for(int i=(x);i<=(y);i++)21 #define for3(i,x,y) for(int i=(x);i>=(y);i--)22 using namespace std;23 inline int read()24 {25     int x=0,f=1;char ch=getchar();26     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}27     while(ch>=‘0‘&&ch<=‘9‘){x=10*x+ch-‘0‘;ch=getchar();}28     return x*f;29 }30 int n;31 struct rec{ll x,y;double z;}a[maxn];32 inline double kk(rec a,rec b)33 {34     if(b.x==a.x)35     {36         if(b.y<a.y)return -inf;else return inf;37     }38     return (double)(b.y-a.y)/(double)(b.x-a.x);39 }40 inline bool cmp(rec a,rec b)41 {42     return a.z<b.z;43 }44 int main()45 {46     freopen("input.txt","r",stdin);47     freopen("output.txt","w",stdout);48     n=read();49     for1(i,n)a[i].x=read(),a[i].y=read();50     ll ans=0;51     for1(i,n-2)52     {53       int k=i;    54       for2(j,i+1,n)if(a[j].x<a[k].x)k=j;55       swap(a[k],a[i]);56       for2(j,i+1,n)a[j].z=kk(a[i],a[j]);57       sort(a+i+1,a+n+1,cmp);    58       ll xx=0,yy=0;59       for2(j,i+1,n)60         {61           ans+=(a[j].x-a[i].x)*yy-(a[j].y-a[i].y)*xx;62           xx+=a[j].x-a[i].x;yy+=a[j].y-a[i].y;63               //cout<<ans<<‘ ‘<<xx<<‘ ‘<<yy<<‘ ‘<<a[j].x<<‘ ‘<<a[j].y<<‘ ‘<<a[j].z<<endl;64         }              65     }    66     printf("%lld",abs(ans)/2);67     if(ans&1)puts(".5");else puts(".0");68     return 0;69 }