cf B. Vasily the Bear and Fly

http://codeforces.com/contest/336/problem/B

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <algorithm>

 4 #include <cmath>

 5 using namespace std;

 6 

 7 int main()

 8 {

 9     int m,r;

10     scanf("%d%d",&m,&r);

11     double sum=0;

12     for(int i=0; i<m; i++)

13     {

14         if(i==0) sum+=2*r*m;

15         else if(i==1) sum+=(2*r+sqrt(2)*r)*(2*m-2);

16         else

17         {

18             sum+=(2*m-2*i)*(((2*i-2)+sqrt(8))*r);

19         }

20     }

21     printf("%lf\n",sum/m/m);

22     return 0;

23 }

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