#include <cstdio>
#include <cstring>
#include <cmath>
#include <set>
#include <map>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define foreach(it,v) for (__typeof((v).end()) it = (v).begin(); it != (v).end(); it++)
typedef long long ll;
const int N = 1005;

bool check(int n, double x[], double y[], double r[]) {
	const double eps = 1e-6;
	const double INF = 1e6;
	double L = -INF, R = INF, U, D;
	rep (i, n) {
		L = max(L, x[i] - r[i]);
		R = min(R, x[i] + r[i]);
	}
	if (L + eps < R) {
		rep (o, 40) {
			double mid = (L + R) / 2;
			D = -INF; U = INF;
			int j, k;
			rep (i, n) {
				double d = sqrt(r[i] * r[i] - (x[i] - mid) * (x[i] - mid));
				if (y[i] + d < U) {
					U = y[i] + d;
					j = i;
				}
				if (y[i] - d > D) {
					D = y[i] - d;
					k = i;
				}
			}
			if (D + eps < U) return 1;
			if ((x[j] - x[k]) * (x[j] - x[k]) + (y[j] - y[k]) * (y[j] - y[k]) + eps > (r[j] + r[k]) * (r[j] + r[k])) return 0;
			if (x[j] > x[k]) swap(j, k);
			double curx = x[j] + r[j] / (r[j] + r[k]) * (x[k] - x[j]);
			if (curx < mid) {
				R = mid;
			} else {
				L = mid;
			}
		}
		return 0;
	} else {
		return 0;
	}
}

int main() {
#ifdef debug
	freopen("in", "r", stdin);
#endif
	int n;
	static double x[N], y[N], r[N];
	while (scanf("%lf%lf%lf", x, y, r + 1) != EOF) {
		scanf("%d", &n);
		for (int i = 1; i <= n; i++) {
			scanf("%lf%lf", x + i, y + i);
			r[i] = r[1];
		}
		r[0] = 1e6;
		if (!check(n + 1, x, y, r)) {
			puts("X");
		} else {
			double L = 0, R = 1e6, mid;
			rep (i, 40) {
				mid = (L + R) / 2;
				r[0] = mid;
				if (check(n + 1, x, y, r)) {
					R = mid;
				} else {
					L = mid;
				}
			}
			printf("%.2f\n", L);
		}
	}
}