#define  _CRT_SECURE_NO_WARNINGS
#pragma comment (linker, "/STACK:526000000")

#include "bits/stdc++.h"

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144 * 4LL
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

void init() {
    iostream::sync_with_stdio(false);
    cout << fixed << setprecision(20);
}

#define int ll
vector<int> inputs;
vector<int> mans[200000];
int n, m;
int check(int border) {
    vector<int> color;
    REP(i, n) {
        color.push_back(0);
    }
    {
        priority_queue<pair<int, int>> next;
        int cnt = 0;
        int now = 0;
        for (int q = n - 1; q >= 0; --q) {
            for (auto x : mans[q]) {
                next.push(mp(q - (x+border) / border, x % border));
                now += x + border;
                cnt++;
            }
            while (next.empty() == false&&next.top().first == q) {
                cnt--;
                now -= next.top().second;
                next.pop();
            }
            now -= cnt * border;
            color[q] += now;
        }
    }
    {
        priority_queue<pair<int, int>> next;
        int cnt = 0;
        int now = 0;
        for (int q = 1; q < n; ++q) {
            for (auto x : mans[q-1]) {
                next.push(mp(-((q-1) + (x + border ) / border), x % border));
                now += x;
                cnt++;
            }
            while (next.empty() == false&&next.top().first == -q) {
                cnt--;
                now -= next.top().second;
                next.pop();
            }
            now -= cnt * border;
            color[q] += now;
        }
    }
    REP(i, n) {
        if (inputs[i] <= color[i]) return 0;
    }
    return 1;
}
void solve() {
    cin >> n >> m;
    REP(i, n) {
        int a;
        cin >> a;
        inputs.push_back(a);
    }
    int hoge = 0;
    REP(i, m) {
        int a, b;
        cin >> a >> b;
        a--;
        mans[a].push_back(b);
        hoge += b;
    }
    //check(2);
    int ok = 1;
    REP(i, inputs.size()) {
        if (hoge >= inputs[i]) {
            ok = 0;
        }
    }
    if (ok == 1) {
        cout << 0 << endl;
        return;
    }
    int bot = 1e9;
    int top = 0;
    while (bot - top > 1) {
        int mid = (top + bot) / 2;
        if (check(mid)) {
            bot = mid;
        }
        else {
            top = mid;
        }
    }
    if (bot == 1e9) bot = -1;
    cout << bot << endl;
}

#undef int
int main() {
    init();

    solve();
}