#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main(){
    long long n, k, c, d;
    cin>>n>>k>>c>>d;
    vector <long long> a(n);
    vector <long long> b(n);
    for (long long i=0; i<n; i++){
        cin>>a[i]>>b[i];
    }
    if (n==1){
        cout<<d*(b[0]-a[0])<<endl;
        return 0;
    }
    vector <long long> dist(n-1);
    for (long long i=0; i<n-1; i++){
        dist[i]=a[i+1]-b[i];
    }
    sort(dist.begin(), dist.end());
    reverse(dist.begin(), dist.end());
    vector <long long> prefix(n-1);
    prefix[0]=dist[0];
    for (long long i=1; i<n-1; i++){
        prefix[i]=prefix[i-1]+dist[i];
    }
    vector <long long> ans(n);
    for (long long i=0; i<n; i++){
        ans[i]+=c*i;
        if ((i+1)*k<2){
            ans[i]+=prefix[n-2]*d;
            continue;
        }
        if ((i+1)*k>=n){
            continue;
        }
        ans[i]+=d*(prefix[n-2]-prefix[(i+1)*k-2]);
    }
    long long minimum=ans[0];
    for (long long i=0; i<n; i++){
        if (ans[i]<minimum){
            minimum=ans[i];
            //cout<<i<<"\n";
        }
    }
    for (long long i=0; i<n; i++){
        minimum+=d*(b[i]-a[i]);
    }
    cout<<minimum<<endl;
}