//TLE解
#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
void add(int s[], int l, int r, int a, int d) {
	for (int i = l; i < r; i++) {
		s[i] += a;
		a += d;
	}
}

bool check(int c) {
	int i;
	static int s[100000];
	
	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(s, l, x[i], w[i] - c * (x[i] - l), c);
		add(s, x[i], r, w[i], -c);
	}
	
	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;
}