#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<vector>
#include<algorithm>
#include<map>
#include<set>
#include<queue>
#include<stack>
using namespace std;
typedef long long ll;
#define fi first
#define se second
#define mp make_pair
const int inf=1e9+7;
const ll mod=1e9+7;

int main() {
    double N, K;
    cin>>N>>K;
    double ue = 0;
    for(double i=0;i<=N;++i){
        ue += i * K;
    }
    cout<<setprecision(15)<<ue/K/(N+1)<<endl;
}