#include <iostream>

int main(void){
  using namespace std;
  int n,k,s=0;

  cin >> n >> k;
  //cout << n << " " << k;

  for(int i=1;i<n+1;i++){
    s += i*k;
  }

  cout << (double)(s/n/k) << endl;  
  return 0;    
}