#include <iostream>
#include <cstdio>

using namespace std;

typedef struct {
  double x;
  double a;
} linear;

int main()
{
  int n;
  linear l[1000];
  
  // *****input*****
  cin >> n;
  cin >> l[0].a;

  l[0].x = 0;
  l[1].a = 0;
  l[1].x = 1;

  // *****solve*****
  for(int i=0; i<n-2; i++) {
    l[i+2].x = -l[i].x + 2*l[i+1].x;
    l[i+2].a = -l[i].a + 2*l[i+1].a + 2;
    //cout << l[i+2].x << " " << l[i+2].a << endl;
  }

  double maxx = -1.0e100;
  for(int i=1; i<n; i++) {
    //cout << (-((double)l[i].a)/l[i].x) << endl;
    maxx = max(maxx, -((double)l[i].a)/l[i].x);
  }

  double ans;

  ans = l[n-1].a + maxx*l[n-1].x;

  printf("%.2lf\n", ans);

  return 0;
}