#include <iostream>
#include <queue>
#include <cmath>

int l, w, n, x1[105], y_1[105];

double dist (int a, int b) {
	double x = std::abs(x1[a] - x1[b]), y = abs(y_1[a] - y_1[b]);
	return std::sqrt (x * x + y * y) / 2.0;
}

bool can (double p_dist) {
	std::queue <int> q;

	for (int i = 0; i < n; i++)
		if (y_1[i] - (int)p_dist <= 0)
			q.push (i);

	int v, seen[105] = {0};

	while (!q.empty()) {
		v = q.front(), q.pop();
		if (y_1[v] + (int)p_dist >= w)
			return false;

		for (int i = 0; i < n; i++) {
			if (i == v || seen[i])
				continue;
			if (dist(v, i) <= p_dist) {
				seen[i] = 1;
				q.push (i);
			}
		}
	}

	return true;
}


int main() {
	std::cin >> l >> w >> n;

	for (int i = 0; i < n; i++)
		std::cin >> x1[i] >> y_1[i];

	double left = 0, right = w, mid;

	while (std::abs(left - right) > 1e-6) {
		mid = (left + right) / 2;

		if (can(mid))
			left = mid;
		else
			right = mid;
	}
	printf ("%0.5f", mid);


}