C++ :

#include <stdio.h>
#include <queue>
#include <algorithm>
using namespace std;
struct stop
{
	int zhan;
	int you;
}s[10005];
int n,p,l;
priority_queue <int> que;
bool comp(stop s1,stop s2)
{
	return s1.zhan<s2.zhan;
}
int solve()
{
	int ans=0 ,pos=0,tank=p;
	for(int i=0;i<n;i++)
	{
		int d=s[i].zhan-pos;
		while(tank-d<0)
		{
			if(que.empty())
			{
				return -1;
			}
			tank+=que.top();
			que.pop();
			ans++;
		}
		tank-=d;
		pos=s[i].zhan;
		que.push(s[i].you);
	}
	return ans;
}
int main()
{
	int i,a;
     while(scanf("%d",&n)!=EOF)
     {
     	for(i=0;i<n;i++)
     	{
     		scanf("%d%d",&s[i].zhan,&s[i].you);
     	}
     	scanf("%d%d",&l,&p);
     	for(i=0;i<n;i++)
     	{
     		s[i].zhan=l-s[i].zhan;
     	}
     		s[n].zhan=l;
     	s[n++].you=0;
     	while(!que.empty())
     	que.pop();
     sort(s,s+n,comp);
     a=solve();
     printf("%d\n",a);
 }
 return 0;
}