#include <bits/stdc++.h>
using namespace std;

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define FOR(i, a, b) for (int i = (a); i<(b); ++i)
#define RFOR(i, b, a) for (int i = (b)-1; i>=(a); --i)
#define MP make_pair
#define PB push_back
#define F first
#define S second

typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;

const int N = 1047;
const int INF = 1e9 + 7;
int dp[N][N];
bool ok[N];

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	FOR(i, 0, N) FOR(j, 0, N) dp[i][j] = INF;
	dp[0][0] = 0;
	int d, n;
	cin >> n >> d;
	vector<PII> v(n);
	FOR(i, 0, n) cin >> v[i].first >> v[i].second;
	FOR(i, 0, d)
	{
		int a;
		cin >> a;
		ok[a] = 1;
	}
	FOR(i, 0, n)
	{
		FOR(j, 0, i + 1)
		{
			if (!ok[i + 1] || dp[i][j] + v[i].first == 0)
				dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + v[i].first);
			if (!ok[i + 1] || dp[i][j] + v[i].first - v[i].second <= 0)
				dp[i + 1][j + 1] = min(dp[i + 1][j + 1], max(0, dp[i][j] + v[i].first - v[i].second));
		}
	}
	FOR(i, 0, n + 1)
	{
		if (dp[n][i] != INF)
		{
			cout << i << '\n';
			return 0;
		}
	}
	cout << -1 << '\n';
		
	
	
	return 0;
}