#include <bits/stdc++.h>
using namespace std;
const int N = 50050;
const long double eps = 1e-16, eps2 = 1e-16;
int i, j, k, n, m, ch, ff, nls;
int _last[N], _next[N], p[N], T[N << 1];
long double l, r, mid;
long double Ls[N << 1];
struct cc {
  long double a, b;
} A[N];
struct dd {
  long double l, r;
  int x, q;
} B[N];
int R() {
  int x = 0;
  ff = 0;
  ch = getchar();
  while (ch < '0' || '9' < ch) {
    if (ch == '-') ff = 1;
    ch = getchar();
  }
  while ('0' <= ch && ch <= '9') x = x * 10 + ch - '0', ch = getchar();
  if (ff) x = -x;
  return x;
}
bool cmp(const dd &a, const dd &b) {
  if (a.l == b.l) return a.r < b.r;
  return a.l < b.l;
}
bool cmpr(const dd &a, const dd &b) {
  if (a.r == b.r) return a.l < b.l;
  return a.r < b.r;
}
bool cmpl(const dd &a, const dd &b) {
  if (a.l == b.l) return a.r < b.r;
  return a.l < b.l;
}
int Lb(int x) { return x & -x; }
void T_add(int x, int ad) {
  for (; x <= nls + 1; x += Lb(x)) T[x] += ad;
}
int T_query(int x) {
  int y = 0;
  for (; x; x -= Lb(x)) y += T[x];
  return y;
}
long double ab(long double x) {
  if (x < 0) return -x;
  return x;
}
int find(long double x) {
  int l = 1, r = nls, mid;
  while (l < r) {
    mid = (l + r) >> 1;
    if (ab(x - Ls[mid]) < eps) return mid;
    if (x < Ls[mid])
      r = mid - 1;
    else
      l = mid + 1;
  }
  return l;
}
bool check(long double x) {
  int i, l, r, nm = 0, ans = 0, tt = 0;
  long double d, tx1, tx2, t;
  for (i = 1; i <= n; i++) {
    d = -A[i].b * A[i].b + A[i].a * A[i].a * x * x + x * x;
    if (d < -eps) continue;
    if (d < eps) {
      tx1 = -A[i].a * A[i].b / (A[i].a * A[i].a + 1);
      nm++;
      B[nm].l = B[nm].r = atan2(tx1, A[i].a * tx1 + A[i].b);
      Ls[++tt] = B[nm].l;
      B[nm].x = i;
    } else {
      d = sqrt(d);
      tx1 = (-A[i].a * A[i].b - d) / (A[i].a * A[i].a + 1);
      tx2 = (-A[i].a * A[i].b + d) / (A[i].a * A[i].a + 1);
      nm++;
      B[nm].l = atan2(tx1, A[i].a * tx1 + A[i].b);
      B[nm].r = atan2(tx2, A[i].a * tx2 + A[i].b);
      if (B[nm].l > B[nm].r) {
        t = B[nm].l;
        B[nm].l = B[nm].r;
        B[nm].r = t;
      }
      Ls[++tt] = B[nm].l;
      Ls[++tt] = B[nm].r;
      B[nm].x = i;
    }
  }
  sort(B + 1, B + nm + 1, cmp);
  sort(Ls + 1, Ls + tt + 1);
  Ls[tt + 1] = 1e20;
  nls = 0;
  for (i = 1; i <= tt; i++)
    if (Ls[i] + eps2 < Ls[i + 1]) Ls[++nls] = Ls[i];
  for (i = 1; i <= nls + 1; i++) T[i] = 0;
  for (i = 1; i <= nm; i++) {
    l = find(B[i].l);
    r = find(B[i].r);
    ans += T_query(l) - T_query(r + 1);
    T_add(l, 1);
    T_add(r + 1, -1);
  }
  return ans >= m;
}
long double Js(long double x) {
  int i, j, k, nm = 0, t1, t2, num = 0, tt;
  long double a, b, d, tx1, tx2, t, ans = 0;
  for (i = 1; i <= n; i++) {
    d = -A[i].b * A[i].b + A[i].a * A[i].a * x * x + x * x;
    if (d < -eps) continue;
    if (d < eps) {
      tx1 = -A[i].a * A[i].b / (A[i].a * A[i].a + 1);
      nm++;
      B[nm].l = B[nm].r = atan2(tx1, A[i].a * tx1 + A[i].b);
      B[nm].x = i;
    } else {
      d = sqrt(d);
      tx1 = (-A[i].a * A[i].b - d) / (A[i].a * A[i].a + 1);
      tx2 = (-A[i].a * A[i].b + d) / (A[i].a * A[i].a + 1);
      nm++;
      B[nm].l = atan2(tx1, A[i].a * tx1 + A[i].b);
      B[nm].r = atan2(tx2, A[i].a * tx2 + A[i].b);
      if (B[nm].l > B[nm].r) {
        t = B[nm].l;
        B[nm].l = B[nm].r;
        B[nm].r = t;
      }
      B[nm].x = i;
    }
  }
  sort(B + 1, B + nm + 1, cmpr);
  for (i = 1; i <= nm; i++) _last[i] = i - 1, _next[i] = i + 1, B[i].q = i;
  _next[0] = 1;
  _last[nm + 1] = nm;
  sort(B + 1, B + nm + 1, cmpl);
  for (i = 1; i <= nm; i++) p[B[i].q] = i;
  for (i = nm; i > 1; i--) {
    j = B[i].q;
    _next[_last[j]] = _next[j];
    _last[_next[j]] = _last[j];
  }
  for (i = 2; i <= nm; i++) {
    j = B[i].q;
    _next[_last[j]] = j;
    _last[_next[j]] = j;
    for (j = _last[j]; j; j = _last[j]) {
      k = p[j];
      if (B[k].r + eps2 < B[i].l) break;
      if (B[i].r + eps2 > B[k].r) {
        t1 = B[i].x;
        t2 = B[k].x;
        a = (A[t2].b - A[t1].b) / (A[t1].a - A[t2].a);
        b = A[t1].a * a + A[t1].b;
        t = sqrt(a * a + b * b);
        if (t + eps < x) ans += t, num++;
        assert(num <= m);
      }
    }
  }
  ans += x * (m - num);
  return ans;
}
int main() {
  n = R();
  long double x, y;
  x = 0.001 * R();
  y = 0.001 * R();
  m = R();
  for (i = 1; i <= n; i++) {
    A[i].a = 0.001 * R();
    A[i].b = 0.001 * R();
    A[i].b += A[i].a * x - y;
  }
  l = 0.0;
  r = 1e10;
  for (i = 1; i <= 100; i++) {
    mid = 0.5 * (l + r);
    if (check(mid))
      r = mid;
    else
      l = mid;
  }
  printf("%.9lf\n", (double)Js(l));
}
