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求任意多边形的面积
#include<cstdio>
#include<cstring>
#include<cctype>
#include<cmath>
#include<algorithm>
#define maxn 105
using namespace std;
int n;
struct Point{
double x,y;
Point(){}
Point(double a,double b){x=a,y=b;}
}P[maxn];
typedef Point Vector;
Vector operator -(const Point A,const Point B){ return Point(A.x-B.x,A.y-B.y); }
double Cross(Point A,Point B){
return A.x*B.y-A.y*B.x;
}
int main(){
scanf("%d",&n);
for(int i=1;i<=n;i++) scanf("%lf%lf",&P[i].x,&P[i].y);
double ans=0;
for(int i=2;i<n;i++)
ans+=Cross(P[i+1]-P[1],P[i]-P[1]);
printf("%.1lf",fabs(ans)/2);
}