[HDU]2036:改革春风吹满地

 1 #include<map>
 2 #include<set>
 3 #include<list>
 4 #include<deque>
 5 #include<cmath>
 6 #include<queue>
 7 #include<stack>
 8 #include<vector>
 9 #include<cstdio>
10 #include<complex>
11 #include<cstring>
12 #include<cstdlib>
13 #include<iostream>
14 #include<algorithm>
15 #define LL long long
16 #define db double
17 #define RG register
18 #define maxx 110
19 using namespace std;
20 struct point{
21   LL x,y;
22   point() {}
23   point(LL _x,LL _y):x(_x),y(_y) {}
24   LL operator ^ (const point a) const{
25     return x*a.y-y*a.x;
26   }
27 }a[maxx];
28 int n;
29 int main(){
30   while(scanf("%d",&n)&&n){LL x,y;
31     scanf("%lld%lld",&x,&y);LL xx,yy;
32     for(int i=2;i<=n;++i)scanf("%lld%lld",&xx,&yy),a[i]=point(xx-x,yy-y);
33     db sz=0;
34     for(int i=2;i<n;++i)sz+=a[i]^a[i+1];
35     printf("%.1lf\n",sz/2);
36   }
37   return 0;
38 }