#define _CRT_SECURE_NO_DEPRECATE
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <ctime>
#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 all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
const ld epsylon = 1e-9;
typedef unsigned int ui;
inline long double get_time(){   
	return (long double)clock()/CLOCKS_PER_SEC;
};

int n, k, l;
int a[100001];

int main()
{
	freopen("metro.in","r",stdin);
	freopen("metro.out","w",stdout);
	//program
	scanf("%d %d %d", &l, &k, &n);
	int minai = 1000000;
	int mini = -1;
	for (int i = 0; i < n; ++i)
	{
		scanf("%d", &a[i]);
		if (abs(k-a[i]) < minai)
		{
			minai = abs(k-a[i]);
			mini = i;
		}
	}
	double avg = (double) 0.5;
	cout << fixed << setprecision(6) <<
		avg + (double) minai << endl;
	//end program
	return 0;
}