#include<cstdio>

#include<queue>

#include<algorithm>

using namespace std;

priority_queue<int> que;

struct Stop {

    int dis,fuel;

}stop[];

bool cmp (Stop a,Stop b) {

    if(a.dis==b.dis) return a.fuel>b.fuel;

    return a.dis<b.dis;

}

int main() {

    int n,L,p;

    while(~scanf("%d",&n)) {

        for(int i=;i<n;i++) {

            scanf("%d%d",&stop[i].dis,&stop[i].fuel);

        }

        scanf("%d%d",&L,&p);

        for(int i=;i<n;i++) {

            stop[i].dis=L-stop[i].dis;

        }

        sort(stop,stop+n,cmp);

        stop[n].dis=L;stop[n].fuel=;

        int cnt=,pos=,flag=;

        for(int i=;i<=n;i++) {

            int d=stop[i].dis-pos;

            while(p<d) {

                if(!que.empty()) {

                    p+=que.top();

                    que.pop();

                    cnt++;

                }else break;

            }

            if(p<d) {

                flag=;

                puts("-1");break;

            }

            p-=d;

            que.push(stop[i].fuel);

            pos=stop[i].dis;

        }

        if(flag==) printf("%d\n",cnt);

        while(!que.empty()) que.pop();

    }

    return ;

}