#include <cstdio>

using namespace std;

int a[100006];

inline int fabss(int x)
{
    if ( x < 0 ) return -x;
    return x;
}

int main()
{
   freopen ( "metro.in", "r", stdin );
	freopen ( "metro.out", "w", stdout );
    
    int result=0,begin,end,expected,up,down;
    
    int l,k,n,i;
    scanf ("%d%d%d",&l,&k,&n);
    for ( i=1; i<=n; i++ )
    {
        scanf ("%d",&a[i]);
        a[i] = a[i]<<1;
    }
    
    for ( i=1; i<=n; i++ )
    {
        if ( i == 1 ) begin = 0;
        else begin = (a[i] + a[i-1]) /2;
        if ( i == n ) end = l<<1;
        else end = (a[i] + a[i+1]) /2;
        expected = fabss((k<<1)-a[i])<<2;
        up = a[i] - begin;
        down = end - a[i];
        expected += ( up*up + down*down );
        
        result += expected;
    }
    
    printf ("%.6f\n",(float)result/(float)(l*8));
}