#include <bits/stdc++.h>
using namespace std;
const int N = 5e4 + 5;
const double eps = 1e-10;
int n, m, cnt, v[N], tr[N << 1];
double a[N], b[N];
pair<double, int> e[N << 1];
inline int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = (x << 3) + (x << 1) + (ch ^ 48);
    ch = getchar();
  }
  return x * f;
}
int lowbit(int x) { return x & (-x); }
void add(int x, int y) {
  while (x <= cnt) tr[x] += y, x += lowbit(x);
}
int query(int x) {
  int res = 0;
  while (x) res += tr[x], x -= lowbit(x);
  return res;
}
void work(double r) {
  cnt = 0;
  for (int i = 1; i <= n; i++) {
    double a = ::a[i], b = ::b[i],
           d = 4.0 * (a * a * b * b - (a * a + 1.0) * (b * b - r * r));
    if (d < eps) continue;
    d = sqrt(d);
    double x = (-2.0 * a * b - d) / ((a * a + 1.0) * 2.0),
           xx = (-2.0 * a * b + d) / ((a * a + 1.0) * 2.0);
    double y = a * x + b, yy = a * xx + b;
    e[++cnt] = make_pair(atan2(y, x), i);
    e[++cnt] = make_pair(atan2(yy, xx), i);
  }
  sort(e + 1, e + cnt + 1);
  for (int i = 1; i <= cnt; i++) v[e[i].second] = i;
}
int check(double r) {
  work(r);
  for (int i = 1; i <= cnt; i++) tr[i] = 0;
  long long ans = 0;
  for (int i = 1; i <= cnt; i++)
    if (v[e[i].second] != i) {
      ans += query(v[e[i].second]) - query(i - 1);
      add(v[e[i].second], 1);
    }
  return ans >= m;
}
double calc(int i, int j) {
  double x = (b[j] - b[i]) / (a[i] - a[j]), y = a[i] * x + b[i];
  return sqrt(x * x + y * y);
}
double calc(double r) {
  work(r);
  set<pair<double, int> > s;
  double ans = 0;
  int tot = 0;
  for (int i = 1; i <= cnt; i++)
    if (v[e[i].second] != i) {
      auto it = s.upper_bound(make_pair(i + 1, 0));
      while (it != s.end() && it->first < v[e[i].second])
        ans += calc(e[i].second, it->second), it++, tot++;
      s.insert(make_pair(v[e[i].second], e[i].second));
    }
  return ans + (m - tot) * r;
}
int main() {
  n = read();
  double p = (double)read() / 1000.0, q = (double)read() / 1000.0;
  m = read();
  for (int i = 1; i <= n; i++) {
    a[i] = (double)read() / 1000.0;
    b[i] = (double)read() / 1000.0;
    b[i] += a[i] * p - q;
  }
  double l = 0, r = 1e10;
  for (int i = 1; i <= 100; i++) {
    double mid = (l + r) / 2.0;
    if (check(mid))
      r = mid;
    else
      l = mid;
  }
  printf("%.10f\n", max((double)0, calc(r)));
  fclose(stdin);
  fclose(stdout);
  return 0;
}
