#include <bits/stdc++.h>
using namespace std;
int read() {
  char ch;
  int sig = 1;
  for (ch = getchar(); ch < '0' || ch > '9'; ch = getchar())
    if (ch == '-') sig = -1;
  int x = ch - '0';
  for (ch = getchar(); ch >= '0' && ch <= '9'; ch = getchar())
    x = x * 10 + ch - '0';
  return x * sig;
}
const int N = 5e4 + 5;
const double eps = 1e-10, pi = acos(-1);
int n, m, t, v[N];
double p, q, a[N], b[N];
pair<double, int> s[N << 1];
void solve(double r) {
  t = 0;
  for (int i = 1; i <= n; i++) {
    double a = ::a[i], b = ::b[i];
    double d = 4 * a * a * b * b - 4 * (a * a + 1) * (b * b - r * r);
    if (d < eps) continue;
    d = sqrt(d);
    double x1 = (-2 * a * b - d) / (2 * (a * a + 1));
    double x2 = (-2 * a * b + d) / (2 * (a * a + 1));
    double y1 = a * x1 + b, y2 = a * x2 + b;
    s[++t] = make_pair(atan2(y1, x1), i), s[++t] = make_pair(atan2(y2, x2), i);
    v[i] = 0;
  }
  sort(s + 1, s + t + 1);
  for (int i = t; i >= 1; i--)
    if (!v[s[i].second]) v[s[i].second] = i;
}
int tr[N << 1];
void I(int x) {
  for (; x <= t; x += x & -x) tr[x]++;
}
int Q(int x) {
  int ret = 0;
  for (; x; x -= x & -x) ret += tr[x];
  return ret;
}
bool chk(double r) {
  solve(r);
  for (int i = 1; i <= t; i++) tr[i] = 0;
  long long ret = 0;
  for (int i = 1; i <= t; i++)
    if (v[s[i].second] != i) {
      ret += Q(v[s[i].second]) - Q(i - 1);
      I(v[s[i].second]);
    }
  return ret >= m;
}
double dist(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);
}
set<pair<int, int> > st;
double calc(double r) {
  solve(r);
  double ret = 0;
  int cnt = 0;
  st.clear();
  for (int i = 1; i <= t; i++)
    if (v[s[i].second] != i) {
      set<pair<int, int> >::iterator it = st.upper_bound(make_pair(i + 1, 0));
      while (it != st.end() && (it->first) < v[s[i].second])
        ret += dist(s[i].second, it->second), ++it, ++cnt;
      st.insert(make_pair(v[s[i].second], s[i].second));
    }
  return ret + (m - cnt) * r;
}
int main() {
  n = read();
  p = read() / 1e3;
  q = read() / 1e3;
  m = read();
  for (int i = 1; i <= n; i++)
    a[i] = read() / 1e3, b[i] = read() / 1e3 + a[i] * p - q;
  double l = 0, r = 1e10;
  for (int i = 1; i <= 100; i++) {
    double mid = (l + r) * 0.5;
    if (chk(mid))
      r = mid;
    else
      l = mid;
  }
  printf("%.10lf\n", max(0.0, calc(r)));
  return 0;
}
