不知道為什麼原來的思路就是錯的 ，那也貼上程式碼，希望大牛指點

#include<stdio.h>
#include<iostream>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<queue>
#include<stack>
#include<set>
#include<map>
#include<sstream>
#include<time.h>
#include<malloc.h>

using namespace std;

int n;
int pp,l;
struct qq
{
    int ps;
    int ml;
}p[10100];

bool cmp (qq a ,qq b)
{
    return a.ps < b.ps;
}

int main ()
{
    while (scanf ("%d",&n)!=EOF)
    {
        priority_queue<int,vector<int>,less<int> >q;

        for(int i=0;i<n;i++)
        {
            scanf("%d %d",&p[i].ps , &p[i].ml );
        }

        scanf("%d %d",&l,&pp);

        sort(p,p+n,cmp);

        p[n].ps = l;
        p[n].ml = 0;

        int ans =0;
        int pos =0;
        int tank=pp;
        int ok=1;
        for(int i=0 ;i<n;i++)
        {
            int d =p[i].ps - pos;

            while (tank - d <0)
            {
                if ( q.empty() )
                {
                    ok=0;
                    break;
                }
                tank+=q.top();
                q.pop();
                ans ++;
            }

            tank-=d;
            pos = p[i].ps ;
            q.push( p[i].ml );
        }
        if (ok)
            printf("%d\n",ans);
        else
            puts("-1");

    }
    return 0;
}
 

#include<stdio.h>
#include<iostream>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<queue>
#include<stack>
#include<set>
#include<map>
#include<sstream>
#include<time.h>
#include<malloc.h>

using namespace std;

int n;
int pp,l;

struct qq
{
    int ps;
    int ml;
}p[10100];

bool cmp (qq a ,qq b)
{
    return a.ps > b.ps;
}

int main ()
{
    while (scanf ("%d",&n)!=EOF)
    {
        priority_queue<int,vector<int>,less<int> >q;

        for(int i=0;i<n;i++)
        {
            scanf("%d %d",&p[i].ps , &p[i].ml );
        }

        scanf("%d %d",&l,&pp);

        sort(p,p+n,cmp);

        p[n].ps = l;
        p[n].ml = 0;
        int pos = 0;
        int ans = 0;
        q.push(pp);
        while ( l>0 && !q.empty() )
        {
            ans ++;
            int t = q.top();
            q.pop();
            l-=t;
            while ( pos < n && l<= p[pos].ps )
            {
                q.push( p[pos++].ml );
            }

        }
        
        if (l<=0)
            printf("%d\n",ans-1);
        else
            puts("-1");

    }
    return 0;
}