#include <bits/stdc++.h>
using namespace std;
priority_queue<int> Q;
struct node{int x, u;}a[11000];
bool cmp(node n1, node n2){ return n1.x < n2.x;}

int n, L, P;

bool pd()
{
    int np = P;
    for(int i = 2; i <= n; i++)
    {
        np -= (a[i].x - a[i-1].x);
        if(np < 0) return 0;
        np += a[i].u;
    }
    return 1;
}
int main()
{
    scanf("%lld", &n);
    for(int i = 1; i <= n; i++)
        scanf("%d%d", &a[i].x, &a[i].u);

    scanf("%d%d", &L, &P);
    for(int i = 1; i <= n; i++)
        a[i].x = L - a[i].x;
    a[n+1].x = 0, a[n+1].u = 0;
    a[n+2].x = L, a[n+2].u = 0;
    n += 2;
    sort(a + 1, a + n + 1, cmp);
    
    if(pd() == 0){ printf("-1"); return 0; }
    
    int ans = 0, np = P;
    for(int i = 2; i <= n; i++)
    {
        np -= (a[i].x - a[i-1].x);
        while(np < 0)
        {
            np += Q.top();
            Q.pop();
            ans++;
        }
        Q.push(a[i].u);
    }
    printf("%d\n", ans);
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
priority_queue<int>Q;
struct node{int x,u;}a[11000];
bool cmp(node n1,node n2){ return n1.x<n2.x;}

int n,L,P;

bool pd()
{
    int np=P;
    for(int i=2;i<=n;i++)
    {
        np-=(a[i].x-a[i-1].x);if(np<0) return 0;
        np+=a[i].u;
    }
    return 1;
}
int main()
{
    scanf("%lld",&n);
    for(int i=1;i<=n;i++)scanf("%d%d",&a[i].x,&a[i].u);

    scanf("%d%d",&L,&P);
    for(int i=1;i<=n;i++)a[i].x=L-a[i].x;
    a[n+1].x=0,a[n+1].u=0;
    a[n+2].x=L,a[n+2].u=0;
    n+=2;
    sort(a+1,a+n+1,cmp);
    
    if(pd()==0){ printf("-1"); return 0; }
    
    int ans=0,np=P;
    for(int i=2;i<=n;i++)
    {
        np-=(a[i].x-a[i-1].x);while(np<0){np+=Q.top();Q.pop();ans++;}
        Q.push(a[i].u);
    }
    printf("%d\n",ans);
    return 0;
}