#include<iostream>
#include<algorithm>
#include<vector>
#include<functional>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<cstdio>
#include<numeric>
using namespace std;
using ll = long long;
int main(){
  int n, k;
  cin >> n >> k;
  double sum = 0, cnt = 0;
  for(int i = 0; i <= n; i++){
    sum += i * k;
    cnt += k;
  }
  printf("%.10f\n", (double)sum / cnt);
  return 0;
}