#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <math.h>

#define ll long long
#define ld long double
#define pll pair<ll, ll>

using namespace std;

const double eps = 0.0001;

ll xl, xr, r, n, xi, yi;
vector<pll> pillars;

class Graph {
public:
	vector<vector<ll>> adj;
	
	Graph(int v) {
		adj.resize(v);
	}

	void addEdge(int u, int w) {
		adj[u].push_back(w);
	}
};

bool canFit(pll firstP, pll secondP, ld R) {
	return sqrt(pow(secondP.first - firstP.first, 2) + pow(secondP.second - firstP.second, 2)) - 2 * r > R * 2;
}

bool checkGraph(ld R, ll s, ll e) {	
	Graph g(n + 2);
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			if (i != j && !canFit(pillars[i], pillars[j], R)) {
				g.addEdge(i, j);
			}
		}

		if (pillars[i].first - r - xl <= R * 2) {
			g.addEdge(i, n);
			g.addEdge(n, i);
		}

		if (pillars[i].first + r + R * 2 >= xr) {
			g.addEdge(i, n + 1);
			g.addEdge(n + 1, i);
		}
	}

	vector<bool> used(g.adj.size());
	queue<ll> q;
	used[s] = true;
	q.push(s);

	while (!q.empty()) {
		ll cur = q.front();
		q.pop();

		for (ll i : g.adj[cur]) {
			if (!used[i]) {
				q.push(i);
				used[i] = true;
				if (i == e) return true;
			}
		}
	}

	return false;
}

ld binSrch() {
	ld lt = 0;
	ld rt = abs(xr) + abs(xl);

	while (rt - lt > eps) {
		ld m = (rt + lt) / 2;
		bool smaller = checkGraph(m, n, n + 1);

		if (smaller) rt = m;
		else lt = m;
	}

	return lt;
}

int main()
{
	cin >> xl >> xr;
	cin >> r;
	cin >> n;

	pillars.resize(n);

	for (int i = 0; i < n; i++) {
		cin >> xi >> yi;
		pillars[i].first = xi;
		pillars[i].second = yi;
	}

	cout << binSrch() * 2;

	return 0;
}