#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <set>

using namespace std;

int n, d;
int m[1000], c[1000];

int main() {
    cin >> n >> d;
    int pointer = 0;
    while (pointer < n) {
        cin >> m[pointer] >> c[pointer];
        pointer++;
    }
    pointer = 0;
    int ret = 0, curr = 0, visit;
    while (pointer++ < d) {
        set<int> s;
        int mess = 0;
        cin >> visit;
        while (visit - 1 >= curr) {
            s.insert(c[curr]);
            mess += m[curr++];
        }
        while (s.size() > 0) {
            mess -= *(s.rbegin());
            ret++;
            if (mess <= 0) break;
        }
        if (mess > 0) {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ret << endl;
    return 0;
}