#include<iostream>
#include<fstream>
#include<sstream>
#include<vector>
#include<cmath>
#include<queue>
#include<string>
#include<iomanip>

using namespace std;

int main(){

    //1084
    double glength,r;

    double pi = 3.1415926535897932384626433832795;

    cin>>glength>>r;

    cout<<setiosflags(ios::fixed)<<setiosflags(ios::showpoint)<<setprecision(3);

    if(glength >= r * 2.0){
        cout<<r*r*pi<<endl;
        return 0;
    }
    if(glength * pow(2.0,0.5) <= r){
        cout<<glength*glength<<endl;
        return 0;
    }
    else{
        double d = pow((pow(r,2.0) - pow(glength/2.0,2.0)),0.5);
        double area = glength/2.0 * d/2.0;
        double arc = pi/4.0 - acos(0.5*glength/r);
        cout<< 8.0*(0.5*arc*r*r+area)<<endl;
        return 0;
    }

}