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 ",sum/m/m); 22 return 0; 23 }