1 /*
2 HDU5979 Convex
3 http://acm.hdu.edu.cn/showproblem.php?pid=5979
4 计算几何 三角形面积公式
5 *
6 *
7 */
8 #include <cstdio>
9 #include <algorithm>
10 #include <cmath>
11 using namespace std;
12 const double Pi=acos(-1.0);
13 int main()
14 {
15 int n,b;
16 //freopen("2.in","r",stdin);
17 double r;
18 while(scanf("%d%d",&n,&b)==2)
19 {
20 r=1.0*b;
21 r=r*r;
22 r=r/2.0;
23 double ans=0.0;
24 double de;
25 while(n--)
26 {
27 scanf("%d",&b);
28 de=1.0*b;
29 ans+=sin(Pi*de/180.0);
30 //printf("%.3lf\n",ans);
31 }
32 ans=ans*r;
33 printf("%.3lf\n",ans);
34 }
35 return 0;
36 }
