import java.util.*;

public class familyvisit_da {

    private void work() {
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        int d = sc.nextInt();
        m = new int[n];
        c = new int[n];
        for (int i = 0; i < n; i++) {
            m[i] = sc.nextInt();
            c[i] = sc.nextInt();
        }

        v = new boolean[n];
        while (d-- > 0) v[sc.nextInt() - 1] = true;

        memo = new int[n][n + 1];
        for (int i = 0; i < n; i++) {
            Arrays.fill(memo[i], -2);
        }

        int ans = -1;
        for (int i = 0; i <= n; i++) {
            if (go(0, i) >= 0) {
                ans = i;
                break;
            }
        }

        System.out.println(ans);
    }

    private static final int oo = Integer.MAX_VALUE / 4;
    private int n;
    private int[] m, c;
    private boolean[] v;
    private int[][] memo;

    private int go(int k, int cn) {
        if (cn < 0) return -1;
        if (k == n) return oo;
        if (memo[k][cn] > -2) return memo[k][cn];

        int ret = -1;
        if (v[k]) {
            if (m[k] == 0 && go(k + 1, cn) >= 0) {
                ret = 0;
            }

            if (go(k + 1, cn - 1) >= 0) {
                ret = Math.max(ret, c[k] - m[k]);
            }
        } else {
            int t = go(k + 1, cn);
            if (t >= 0) {
                ret = Math.max(ret, t - m[k]);
            }

            t = go(k + 1, cn - 1);
            if (t >= 0) {
                ret = Math.max(ret, t + c[k] - m[k]);
            }
        }

        return memo[k][cn] = ret;
    }

    public static void main(String[] args) {
        new familyvisit_da().work();
    }
}