//やりたいことは何となく分かるけど、詰めるのが難しい。
#include <iostream>
#define int long long
#define rep(i, n) for(i = 0; i < n; i++)
using namespace std;

int n, m;
int a[100000];
int x[100000], w[100000];

//区間[l, r)にa, a+d, a+2d, …を足す
//・s1[]では, a, a, a…の足し算を担当（1回階差）
//・s2[]では, 0, d, 2d…の足し算を担当（2回階差）
void add(int s1[], int s2[], int l, int r, int a, int d) {
	//cout << "l = " << l << ", r = " << r << ", a = " << a << ", d = " << d << endl;
	if (r - l <= 0) { return; }
	s1[l] += a;
	s1[r] -= a;
	
	if (r - l <= 1) { return; }
	s2[l + 1] += d;
	s2[r]     -= (r - l) * d;
	s2[r + 1] += (r - l - 1) * d;
}

bool check(int c) {
	int i;
	static int s1[100001];
	static int s2[100002];
	static int s[100000];
	
	rep(i, n) s1[i] = 0;
	rep(i, n) s2[i] = 0;
	rep(i, n) s[i] = 0;
	
	if (c == 0) {
		int sw = 0;
		rep(i, m) sw += w[i];
		rep(i, n) if (sw >= a[i]) return false;
		return true;
	}
	
	rep(i, m) {
		int l = x[i] - (w[i] + c - 1) / c + 1; if (l < 0) l = 0;
		int r = x[i] + (w[i] + c - 1) / c; if (r > n) r = n;
		add(s1, s2, l, x[i], w[i] - c * (x[i] - l), c);
		add(s1, s2, x[i], r, w[i], -c);
	}
	
	rep(i, n) s1[i + 1] += s1[i];
	rep(i, n) s2[i + 1] += s2[i];
	rep(i, n) s2[i + 1] += s2[i];
	rep(i, n) s[i] = s1[i] + s2[i];
	
	//rep(i, n) cout << s1[i] << " "; cout << endl;
	//rep(i, n) cout << s2[i] << " "; cout << endl;
	
	rep(i, n) { if (s[i] >= a[i]) return false; }
	return true;
}

signed main() {
	int i;
	
	cin >> n >> m;
	rep(i, n) { cin >> a[i]; }
	rep(i, m) { cin >> x[i] >> w[i]; x[i]--; }
	
	int ng = -1, ok = 114514, mid;
	while (ok - ng >= 2) {
		mid = (ok + ng) / 2;
		if (check(mid)) ok = mid;
		else ng = mid;
	}
	if (ok >= 114514) cout << "-1" << endl;
	else cout << ok << endl;
	return 0;
}