//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int main(){
  int N, K; cin >> N >> K;
  ll sum = 0, sumc = 0;
  
  for(int i = 0; i <= N; i++){
    sum += i*K;
    sumc += K;
  }
  
  double ans = (double)sum / (double)sumc;
  
  fix(10);
  printl(ans);
  
  return 0;
}