#include <bits/stdc++.h>
#define rep(i,n) for (int i=0; i<(n); ++i)
using namespace std;
using ll=long long;

int main(){
    int n,k,x,y;
    cin>>n>>k>>x>>y;
    vector<int>a(n);
    queue<int>q;
    rep(i,n){
        cin>>a[i];
        a[i]--;
    }
    sort(a.begin(),a.end());
    rep(i,n)q.push(a[i]);
    ll ans=0;
    int now=0;
    while(1){
        while(1){
            if(q.front()<=now)q.pop();
            else break;
            if(q.empty())break;
        }
        if(q.empty())break;
        if((ll)x*q.size()>(ll)y){
            now+=k,ans+=y;
        }
        else{
            now+=k,ans+=(ll)x*q.size();
        }
    }
    cout<<ans<<endl;
}