// Arup Guha
// 3/4/2018
// Solution to 2018 February USACO Silver Problem: Rest Stops

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

public class reststops {

	public static int len;
	public static int n;
	public static long tJohn;
	public static long tBessie;
	public static grass[] eats;

	public static void main(String[] args) throws Exception {

		// Read the data.
		BufferedReader stdin = new BufferedReader(new FileReader("reststops.in"));
		StringTokenizer tok = new StringTokenizer(stdin.readLine());

		// Get basic data.
		len = Integer.parseInt(tok.nextToken());
		n = Integer.parseInt(tok.nextToken());
		tJohn = Long.parseLong(tok.nextToken());
		tBessie = Long.parseLong(tok.nextToken());

		// Get each grass patch.
		eats = new grass[n];
		for (int i=0; i<n; i++) {
			tok = new StringTokenizer(stdin.readLine());
			int x = Integer.parseInt(tok.nextToken());
			int val = Integer.parseInt(tok.nextToken());
			eats[i] = new grass(x, val);
		}
		Arrays.sort(eats);

		long res = 0;
		int curX = 0;

		// Go through each grass spot in order of value.
		for (int i=0; i<n; i++) {

			// We passed this grass, so skip it.
			if (eats[i].x <= curX) continue;

			// How long we can eat here.
			long xDiff = (long)(eats[i].x-curX);
			long tEat = xDiff*(tJohn-tBessie);

			// Value of what we can eat for this time.
			res += tEat*eats[i].value;

			// Update the current x value.
			curX = eats[i].x;
		}

		// Ta da!
		PrintWriter out = new PrintWriter(new FileWriter("reststops.out"));
		out.print(res);
		out.close();
		stdin.close();
	}
}

class grass implements Comparable<grass> {

	public int x;
	public int value;

	public grass(int myx, int myv) {
		x = myx;
		value = myv;
	}

	public int compareTo(grass other) {
		return other.value - this.value;
	}
}