gistfile1.txt

#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);
        }
    }
}