#include<cstdio>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<iostream>
#include<algorithm>
using namespace std;
double a;
int n;
bool C(double x)
{
    int m=floor((-x))/2;
    return a+(m+1)*x+(m+1)*m>=0;
}
int main()
{
    scanf("%d %lf",&n,&a);
    double left=-a,right=0;
    for(int i=0;i<100;i++)
    {
        double mid=(left+right)/2;
        if(C(mid)) right=mid;
        else left=mid;
    }
    printf("%.2lf\n",a+(n-1)*right+(n-2)*(n-1));
    return 0;
}