#include<bits/stdc++.h>
using namespace std;
int main()
{
    double n,k;
    cin>>n>>k;
    double sum = 0.0;
    n = n+1;
    double total = (n*(n+1))/2;
    //cout<<total<<endl;
    total = (1.00/total);
    //cout<<total<<endl;
    for(int i=0; i<n; i++)
    {
        sum+=(k*i);
    }
    printf("%.5f\n",sum*total);
}