#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;

double l,k;
int n;
double a[100002];


int main()
{
	freopen("metro.in", "r", stdin);
	freopen("metro.out", "w", stdout);

	scanf("%lf%lf%d", &l, &k, &n);
	
	a[0]=-1000000;
	for(int i=1; i<=n; i++)
		scanf("%lf", &a[i]);

	if(n==1)
	{
		double res=max(l-a[1], a[1])/2;
		printf("%lf\n", res);
		return 0;
	}

	sort(a, a+n+1);
	int d=upper_bound(a, a+n, k)-a;
	//printf("upper_bound: %lf %lf\n", a[d-1], a[d]);
	double mi=min(fabs(k-a[d]), abs(a[d-1]-k));
	double tmax=fabs(k-a[1])+max(a[1], (a[2]-a[1])/2);
	tmax=max(tmax, fabs(k-a[n])+max(l-a[n], (a[n]-a[n-1])/2));
	for(int i=2; i<n; i++)
		tmax=max(tmax, fabs(k-a[i])+max((a[i+1]-a[i])/2, (a[i]-a[i-1])/2));
	
	//printf("min:%lf max:%lf\n", mi, tmax);
	double time=(tmax+mi)/2;
	printf("%lf\n", time);
	return 0;
}

/*
10 3.5 4
0 10 2.1 5.1
*/