#include <iomanip>
#include <algorithm>
#include <vector>
#include <cstdio>
using namespace std;

int main()
{
	freopen("metro.in", "r", stdin);
	freopen("metro.out", "w", stdout);
	int n;
	double l, k;
	vector<double> v;
	scanf("%lf %lf %d\n", &l, &k, &n);
	v.resize(n);
	for(int i = 0; i < n; i++)
	{
		scanf("%lf ", &v[i]);
	}
	sort(v.begin(), v.end());
	vector<double>::iterator it = upper_bound(v.begin(), v.end(), k);
	if(it == v.end()) it--;
	else if(fabs(*(it - 1) - k) < fabs(*it - k)) it--;
	printf("%.6lf\n", *it + fabs(*it - k));
	return 0;
}