#define _CRT_SECURE_NO_DEPRECATE
#include <iostream> 
#include <string>
#include <vector>
#include <sstream>
#include <queue>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <math.h>
#include <stack>
#include <deque>
#include <numeric>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <bitset>
#define mpair make_pair
#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef long double ld;
const ld epsylon = 1e-9;
double get(double x, double y) {
  return fabs(x - y);
}
int main()
{
  freopen("metro.in","r",stdin);
  freopen("metro.out", "w", stdout);

  int n;
  double k,l;
  cin >> l >> k >> n;
  vector<double> a(n);
  for (int i =0;i<n;++i){
    cin >> a[i];
  }
  sort(all(a));

  double ans = (a[0] * 0.5 + get(a[0], k)) * a[0];
  ans += ((l - a.back())* 0.5 + get(a.back(), k)) * (l - a.back());

  for (int i = 0; i + 1 < n; ++i) {
    ans += ((a[i+1] - a[i])*0.25 + get(a[i], k))*(a[i+1] - a[i])*0.5;
    ans += ((a[i+1] - a[i])*0.25 + get(a[i+1], k))*(a[i+1] - a[i])*0.5;
  }

  printf("%.6lf\n", ans / l);
  return 0;
}