HDU5979 Convex

 /*
  HDU5979 Convex
  http://acm.hdu.edu.cn/showproblem.php?pid=5979
  计算几何 三角形面积公式
  *
  *
  */
 #include <cstdio>
 #include <algorithm>
 #include <cmath>
 using namespace std;
 const double Pi=acos(-1.0);
 int main()
 {
     int n,b;
     //freopen("2.in","r",stdin);
     double r;
     while(scanf("%d%d",&n,&b)==)
     {
         r=1.0*b;
         r=r*r;
         r=r/2.0;
         double ans=0.0;
         double de;
         while(n--)
         {
             scanf("%d",&b);
             de=1.0*b;
             ans+=sin(Pi*de/180.0);
             //printf("%.3lf\n",ans);
         }
         ans=ans*r;
         printf("%.3lf\n",ans);
     }
     return ;
 }