/*

  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 ;

 }