#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;

struct pt{
  typedef pt P;
  double x, y;
	explicit pt(double x=0, double y=0) : x(x), y(y) {}
  // y then x comparison
	bool operator<(P p) const { return tie(y,x) < tie(p.y,p.x); }
	bool operator==(P p) const { return tie(x,y)==tie(p.x,p.y); }
	P operator+(P p) const { return P(x+p.x, y+p.y); }
	P operator-(P p) const { return P(x-p.x, y-p.y); }
	P operator*(int s) const { return P(s*x, s*y); }
	double dot(P p) const { return x*p.x + y*p.y; }
	double dot(P a, P b) const { return (a-*this).dot(b-*this); }
	double cross(P p) const { return x*p.y - y*p.x; }
	double cross(P a, P b) const { return (a-*this).cross(b-*this); }
	double angle() const { return atan2(y, x); }
  double dist2() const { return x*x + y*y; }
  double dist2(P a) const { return (a-*this).dist2(); }
};
ostream& operator<<(ostream &o,const pt &p) {
  return o<<"("<<p.x<<","<<p.y<<")"; }

typedef vector<pt> pol;

pol chull(pol p) {
  sort(p.begin(), p.end()); int top=0, bot=1, n=p.size();
  pol ch(2*n); for (int i = 0, d = 1; i < n && i >= 0; i += d) {
    // If no duplicates, can change <= 0 to < 0 to keep redundant points
    while (top>bot && (ch[top-1]-ch[top-2]).cross(p[i]-ch[top-2])<=0) top--;
    ch[top++] = p[i]; if (i == n-1) d = -1, bot = top;
  } ch.resize(max(1, top-1)); return ch; }

int n, k, d;

const int MAXN = 2001;
ld choose[MAXN][MAXN];

ld chs(int n, int k) {
  if (n<k || k<0 || n < 0) return 0;
  return choose[n][k];
}

int main(){
  cin.tie(0); ios_base::sync_with_stdio(0);
  cout << setprecision(10) << fixed;
  // precompute choose function
  choose[0][0] = choose[1][0] = choose[1][1] = 1;
  for(int i=2;i<MAXN;i++) {
    choose[i][0] = 1;
    for(int j=1;j<=i;j++) {
      choose[i][j] = choose[i-1][j-1] + choose[i-1][j];
    }
  }
  // program start
  cin >> n >> k >> d;
  vector<pt> pts;
  mt19937 rng(0x3ffff);
  for(int i=0;i<n;i++) {
    double x,y;
    cin >> x >> y;
    x+=1e-12*((int)rng()%100000-50000);
    y+=1e-12*((int)rng()%100000-50000);
    pts.emplace_back(x,y);
  }
  double ans = d*M_PI;
  // events, +1 is insertion into halfplane, -1 is removal from halfplane
  for(int i=0;i<n;i++) {
    double ans2 = 0;
    vector<int> ids;
    vector<double> ang(2*n+1);
    for(int j=0;j<n;j++) {
      if (i==j) continue;
      ang[n+j+1] = (pts[j]-pts[i]).angle();
      if (ang[n+j+1] < 0) ang[n+j+1]+=2*M_PI;
      ang[n-(j+1)] = ((pts[j]-pts[i])*-1).angle();
      if (ang[n-(j+1)] < 0) ang[n-(j+1)]+=2*M_PI;
      ids.emplace_back(j+1); // insertion
      ids.emplace_back(-j-1); // deletion
    }
    auto cmp = [&] (int ap, int bp) {
          return ang[n+ap] < ang[n+bp];
        };
    sort(ids.begin(), ids.end(), cmp);
//    for(auto ap: ids) { cerr << pts[(abs(ap)-1)]-pts[i] << " " << (ap>0?1:-1) << " ang " <<ang[n+ap] <<endl; }
    int j = 0;
    // initialize halfplane
    int left = 0;
    for(int steps=0;steps<2*(n-1);steps++,j=(j+1)%ids.size()) {
      int a = abs(ids[j])-1;
      int sa =ids[j]>0?1:-1;
      if ((pts[a]-pts[i]).y*sa < 0 || ((pts[a]-pts[i]).y==0 && (pts[a]-pts[i]).x*sa < 0)) break;
      if (sa==1) left++;
    }
    // sweep events starting from here.
    for(int steps=0;steps<2*(n-1);steps++, j=(j+1)%ids.size()) {
      int a = abs(ids[j])-1;
      int sa =ids[j]>0?1:-1;
//      cerr <<a << " " << pts[a]-pts[i] << " sa:" << sa << " left:" << left << " chs" << n-left-2 << " " << k-2 << " "<<chs(n-left-2, k-2)/chs(n,k) * sqrt(pts[i].dist2(pts[a])) <<endl;
      if (sa == 1) { // count contribution
        ans2 += chs(n-left-2, k-2)/chs(n,k) * sqrt(pts[i].dist2(pts[a]));
        left++;
      }
      else {
        left--;
      }
    }
    //cerr << i << " GOT " << ans2 <<endl;
    ans += ans2;
  }
  cout << ans <<endl;
}