HDU2036 #include<cstdio> #include<cstdlib> #include<iostream> #include<memory.h> #include<algorithm> #include<cmath> using namespace std; int main() { int i,n; double x1,y1,x2,y2,x3,y3; double ans,temp; while(~scanf("%d",&n)){ if(n==0) return 0; ans=0; cin>>x1>>y1>>x2>>y2; for(i=3;i<=n;i++){ cin>>x3>>y3; temp=((x2-x1)*(y3-y1)-(x3-x1)*(y2-y1))*0.5;//思考为什么不要加绝对值 ans+=temp; x2=x3;y2=y3; } printf("%.1lf\n",ans); } return 0; }