#include<bits/stdc++.h>
#define int long long
#define repe(i,l,r) for(int (i)=l;(i)<=r;(i)++)
#define rep(i,n) for(int (i)=1;(i)<=n;(i)++)
#define FOR(i,r,l) for(int (i)=r;(i)>=l;(i)--)
#define INF 0x3f3f3f
#define pii pair<int,int>
#define mpr make_pair
#define pb push_back
#define ALL(v) (v).begin(),(v).end()
#define rsort(v) sort(ALL(v),greater<int>())
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
using namespace std;
int read(){int sum=0,f=1;char c;c=getchar();while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}while(c>='0'&&c<='9'){sum=(sum<<1)+(sum<<3)+(c-'0');c=getchar();}return sum*f;}
void out(int x){if(x<0){x=-x;putchar('-');}if(x>=10)out(x/10);putchar(x%10+'0');}
template <typename T>void die(T s){cout<<s<<endl;exit(0);}
int fast(int a,int b,int P){int res=1;if(P<=0){while(b){if(b&1)res=res*a;a=a*a;b>>=1;}}else{while(b){if(b&1)res=res*a%P;a=a*a%P;b>>=1;}}return res;}
template <typename T>void chkmax(T& a,T b){if(a<b)a=b;return;}
template <typename T>void chkmin(T& a,T b){if(a>b)a=b;return;}
int k,n,cost1,cost2;
struct Point{
	double x,y;
}a[705];
double val[705];
double ansx,ansy;
double ans;
double calc(double x,double y){
	double res=sqrt(x*x+y*y)*cost1;
	for(int i=1;i<=n;i++){
		val[i]=sqrt((x-a[i].x)*(x-a[i].x)+(y-a[i].y)*(y-a[i].y));
	}
	nth_element(val+1,val+k,val+n+1);
	res+=val[k]*cost2;
	return res;
}
const int inf=1e10;
void work(){
	double T=2605;
	double x=ansx,y=ansy;
	while(T>1e-15){
		double nx=(rand()*2-RAND_MAX)*T+x;
		double ny=(rand()*2-RAND_MAX)*T+y;
		double now=calc(nx,ny);
		double de=now-ans;
		if(de<0){
			ansx=x=nx,ansy=y=ny,ans=now;
		}
		else if(exp(-de/T)*RAND_MAX>=1.0*rand()){
			x=nx,y=ny;
		}
		T*=0.9972;
	}
	return;
}
int X[705],Y[705];
signed main(){
	k=read(),n=read(),cost1=read(),cost2=read();
	if(cost1==0)srand(2741);
	else srand(1441);
	rep(i,n){
		X[i]=a[i].x=read(),Y[i]=a[i].y=read();
		ansx+=(double)a[i].x,ansy+=(double)a[i].y;
	}
	if(cost1==0){
		if(*max_element(X+1,X+n+1)==*min_element(X+1,X+n+1)){
			ans=calc(0,0);
			sort(Y+1,Y+n+1);
			for(int i=1;i<=n-k+1;i++)chkmin(ans,calc(a[1].x,(double)(Y[i+k-1]+Y[i])/2.0));
			printf("%.6lf\n",ans);
			return 0;
		}
		if(*max_element(Y+1,Y+n+1)==*min_element(Y+1,Y+n+1)){
			ans=calc(0,0);
			sort(X+1,X+n+1);
			for(int i=1;i<=n-k+1;i++)chkmin(ans,calc((double)(X[i+k-1]+X[i])/2.0,a[1].y));
			printf("%.6lf\n",ans);
			return 0;
		}
	}
	ansx=ansx/n,ansy=ansy/n;
	ans=calc(ansx,ansy);
	int st=clock();
	while(1.0*(clock()-st)/CLOCKS_PER_SEC<=1.95)work();
	printf("%.6lf\n",ans);
	return 0;
}