// Problem: D - Family Visits
// Contest: Virtual Judge - sdtbu组队赛3
// URL: https://vjudge.net/contest/558996#problem/D
// Memory Limit: 1024 MB
// Time Limit: 2000 ms
// Date: 2023-05-21 19:23:19

/**
  * @author  : SDTBU_LY
  * @version : V1.0.0
  * @上联    : ac自动机fail树上dfs序建可持久化线段树
  * @下联    : 后缀自动机的next指针DAG图上求SG函数
**/

#include<iostream>
#include<cstring>
#include<algorithm>
#include<set>

#define MAXN 1010

using namespace std;

typedef long long ll;

int n,d;
int a[MAXN],b[MAXN];

int t[MAXN];

int res=0;


int main()
{
    // ios::sync_with_stdio(false);
    // cin.tie(0),cout.tie(0);
    cin >> n >> d;
    
    for(int i=1;i<=n;i++)
        cin >> a[i] >> b[i];
    
    for(int i=1;i<=d;i++)
        cin >> t[i];
    
    for(int i=1;i<=d;i++)
    {
        int cur=0,deal=0;
        
        multiset<int> s;
        
        for(int j=t[i];j>=t[i-1]+1;j--)
        {
            cur+=a[j];
            if(deal>0)
            {
                int num=min(cur,deal);
                cur-=num;
                deal-=num;
            }
            s.insert(b[j]);
            while(cur>0&&s.size()>0)
            {
                multiset<int>::reverse_iterator maxit=s.rbegin();
                int val=*maxit;
                s.erase(*maxit);
                maxit=s.rbegin();
                int num=min(cur,val);
                cur-=num,val-=num;
                deal+=val;
                res++;
            }
            if(cur>0)
            {
                cout << -1 << endl;
                return 0;
            }
        }
        
        
    }
    
    cout << res << endl;
    
    
    
    
    return 0;
}