#include <bits/stdc++.h>
using namespace std;
const int maxn = 110;
const double Eps = 1e-8;
const double e = 2.7182818284590452353;
struct point {
  double x, y;
};
int n, K;
double E, f[maxn][maxn];
point data[maxn];
void input() {
  scanf("%d%d%lf", &n, &K, &E);
  E = 1 - E * 0.001;
  for (int i = 0; i <= n; i++) scanf("%lf%lf", &data[i].x, &data[i].y);
}
bool check(double r) {
  memset(f, 0, sizeof(f));
  f[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    double d = sqrt(((data[i].x - data[0].x) * (data[i].x - data[0].x)) +
                    ((data[i].y - data[0].y) * (data[i].y - data[0].y))),
           p = 0;
    if (d <= r)
      p = 1;
    else
      p = pow(e, 1 - ((d) * (d)) / ((r) * (r)));
    for (int j = 0; j < n; j++) {
      f[i][j + 1] += f[i - 1][j] * p;
      f[i][j] += f[i - 1][j] * (1 - p);
    }
  }
  double suc = 0;
  for (int i = K; i <= n; i++) suc += f[n][i];
  return (suc > E - Eps);
}
void solve() {
  double s = 0, t = 10000;
  while (t - s > Eps) {
    double mid = (s + t) / 2;
    if (check(mid))
      t = mid;
    else
      s = mid;
  }
  printf("%.8lf\n", s);
}
int main() {
  input();
  solve();
  return 0;
}
