#include<iostream>
#include<algorithm>
using namespace std;
int main() {
	int n,s,a,b,g=0;
	int x[5005],y[5005];
	cin>>n>>s>>a>>b;
	for(int i=1;i<=n;i++){
		cin>>x[i]>>y[i];
		sort(x,x+n);
	while(s>0){
		if(x[i]<=(a+b)){
			g++;
		}
		s-=y[i];
	}
	}
	cout<<s;
}