#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int l, n, f, b;
ll rest[1000001];
int mx[1000001];
ll ans;

int main(){
	freopen("reststops.in", "r", stdin);
	freopen("reststops.out", "w", stdout);
	
	cin >> l >> n >> f >> b;
	int cover = f-b;
	for(int i=0; i<n; i++){
		ll p, t;
		cin >> p >> t;
		rest[p] = t;
	}
	mx[l] = l;
	for(int i=l-1; i>=1; i--){
		if(rest[mx[i+1]]<rest[i]){
			mx[i] = i;
		}else{
			mx[i] = mx[i+1];
		}
	}
	//for(int i=1; i<=l; i++){
		//cout << i << ", " << mx[i] << ", " << rest[mx[i]] << '\n';
	//}
	int pos = mx[1];
	int prev = 0;
	while(pos<=l){
		ans+=(cover*rest[pos]*(pos-prev));
		if(pos>=l) break;
		prev = pos;
		pos = mx[pos+1];
	}
	cout << ans << '\n';
	
	return 0;
}