#include <iostream>
#include <stdio.h>
#include <algorithm>
using namespace std;
typedef long long Int;

struct door
{
    double left,right;
    Int place;
    double toexit;
    double coef;
};

Int ABS(Int a)
{
    if (a<0)
    return a*-1;
    else
    return a;
}

door doors[100001];

bool SortDoorsNoob(door a,door b)
{
    return (a.place<b.place);
}

int main()
{
    freopen("metro.in","r",stdin);
    freopen("metro.out","w",stdout);
    Int l,k,n;
    Int i;
    double N1,N2,P1,P2;
    double totalcoef;
    
    scanf("%I64d %I64d %I64d",&l,&k,&n);
    
    for (i=1;i<=n;i++)
    {
        scanf("%I64d",&doors[i].place);
    }
    sort(doors+1,doors+1+n,SortDoorsNoob);
    for (i=1;i<=n;i++)
    {
        if (i==1)
        {
            doors[i].left=0.0;
        }
        else
        {
            doors[i].left=((double)doors[i-1].place+(double)doors[i].place)/2.0;
        }
        if (i==n)
        {
            doors[i].right=(double)l;
        }
        else
        {
            doors[i].right=((double)doors[i+1].place+(double)doors[i].place)/2.0;
        }
        
        doors[i].toexit=(double)ABS( k-doors[i].place );
    }
    totalcoef=0.0;
    for (i=1;i<=n;i++)
    {
        P1=(double)doors[i].place-doors[i].left;
        N1=P1*1000000.0+1;
        P2=doors[i].right-(double)doors[i].place;
        N2=P2*1000000.0+1;
        
        doors[i].coef=( (N1*P1)/2.0 + (N2*P2)/2.0 + doors[i].toexit*(N1+N2) )/(N1+N2);
        totalcoef=totalcoef+doors[i].coef;
    }
    totalcoef=totalcoef/(double)n;
    
    printf("%4.6f\n",totalcoef);
    return 0;
}<!-- 0.0621s -->