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 }