import java.util.*;
import java.io.*;

public class reststops {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new FileReader("reststops.in"));
        PrintWriter pw = new PrintWriter(new FileWriter("reststops.out"));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int n = Integer.parseInt(st.nextToken());
        int reststops = Integer.parseInt(st.nextToken());
        int fjspeed = Integer.parseInt(st.nextToken());
        int bspeed = Integer.parseInt(st.nextToken());

        int[] maxRests = new int[n];
        int[] bessSpeeds = new int[n];
        int[] rests = new int[n];

        for(int x = 0; x<reststops; x++){
            st = new StringTokenizer(br.readLine());
            int res = Integer.parseInt(st.nextToken());
            int tast = Integer.parseInt(st.nextToken());
            rests[res] = tast;
        }

        int restmax = -1;
        for(int x = n-1; x>=0; x--){
           if(rests[x] > 0){
               if(rests[x] > restmax){
                   maxRests[x] = rests[x];
                   restmax = rests[x];
               }
               else{
                   maxRests[x] = restmax;
               }
           }
        }

        int fjtrack = 0;
        int btrack = 0;
        int tasti = 0;
        /*pw.println(Arrays.toString(maxRests));
        pw.println(Arrays.toString(rests));*/

        for(int x = 1; x<n; x++){
            fjtrack+=fjspeed;
            btrack+=bspeed;
            //pw.println(fjtrack + " " + btrack);
            if(rests[x] > 0){
                if(maxRests[x] == rests[x]) { // if it is a maxrest
                    tasti += rests[x] * (Math.abs(btrack-fjtrack));
                    btrack = fjtrack;
                }
            }
        }
        pw.println(tasti);
        pw.close();

    }
}