1pol

#include<iostream>
#include<cmath>
using namespace std;
int main()
{
   double x,answer=0;
   int n;
   cin>>n>>x;
   for(int i=1;i<=n;++i)
       answer+=cos((2*i-1)*x)/(1<<(2*i-1));
   cout<<answer<<endl;
   return 0;
}