#include<iostream>
#include<cmath>
using namespace std;
int main (){
    int n;
    float s=0;
    float a;
    float x;
    cout <<"x,n";cin>> x  >> n ;
	a=x;
	s=s+a;
    for (int i=2 ; i<=n ;i++)
    {
        a=-(x*x)/(2*(2*i-1)*(i-1));
        s=s+a;
    }
    cout<<s<<endl ;
    system("pause");
    return 0;
}