#include <bits/stdc++.h>

using namespace std;

using ll = long long;

const ll MAXV = 100000;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<ll> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    vector<ll> x(m), w(m);
    for (int i = 0; i < m; ++i) {
        cin >> x[i] >> w[i];
        --x[i];
    }

    ll ng = -1, ok = MAXV + 1;
    while (ok - ng > 1) {
        ll c = (ok + ng) / 2;
        if (c == 0) {
            ll sum = 0;
            for (int i = 0; i < m; ++i) {
                sum += w[i];
            }

            bool f = false;
            for (int i = 0; i < n; ++i) {
                if (sum >= a[i]) {
                    f = true;
                    break;
                }
            }
            (f ? ng : ok) = c;
            continue;
        }

        vector<ll> val(n + 1, 0);
        for (int i = 0; i < m; ++i) {
            ll len = w[i] / c;

            ll i1 = max(x[i] - len, 0LL);
            val[i1] += w[i] % c + c * max(len - x[i], 0LL);
            if (len > 0) val[i1 + 1] += c;

            val[x[i] + 1] += -c * 2;

            ll i2 = x[i] + len;
            if (i2 < n) {
                val[i2 + 1] -= w[i] % c;
                val[i2 + 1] += c;

                if (i2 + 1 < n) {
                    val[i2 + 2] += w[i] % c;
                }
            }
        }

        bool f = false;
        for (int i = 0; i < n; ++i) {
            val[i + 1] += val[i];
            if (val[i] >= a[i]) {
                f = true;
                break;
            }
        }
        (f ? ng : ok) = c;
    }
    cout << (ok > MAXV ? -1 : ok) << "\n";
    return 0;
}