#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

constexpr long long MAX = 5100000;
constexpr long long INF = 1LL << 60;
constexpr int inf = 1 << 28;
constexpr long long mod = 1000000007LL;
//constexpr long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

ll n, m;
vector<ll> a, x, w;

bool check(ll mid) {
	vector<ll> sum(n + 5), L(n + 5), R(n + 5), Lcnt(n + 5), Rcnt(n + 5);
	for (int i = 0; i < m; i++) {
		ll d = w[i] / mid;
		ll left = max(0LL, x[i] - d);
		ll right = min(n - 1, x[i] + d);
		sum[left] += w[i];
		sum[right + 1] -= w[i];
		if (x[i] != 0) L[x[i] - 1]++;
		if (left != 0) L[left - 1]--, Lcnt[left - 1] -= d;
		R[x[i] + 1]++;
		R[right + 1]--, Rcnt[right + 1] -= d;
	}
	for (int i = 0; i <= n; i++) sum[i + 1] += sum[i];
	for (int i = n; i >= 0; i--) L[i] += L[i + 1];
	for (int i = 0; i <= n; i++) R[i + 1] += R[i];
	for (int i = 0; i <= n; i++) L[i] += Lcnt[i], R[i] += Rcnt[i];
	for (int i = n; i >= 0; i--) L[i] += L[i + 1];
	for (int i = 0; i <= n; i++) R[i + 1] += R[i];
	for (int i = 0; i <= n; i++) sum[i] -= (L[i] + R[i]) * mid;
	for (ll i = 0; i < n; i++) {
		//cout << mid << " " << sum[i] << " " << a[i] << endl;
		if (sum[i] > a[i]) return false;
	}
	return true;
}
int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	scanf("%lld %lld", &n, &m);
	a.resize(n); for (int i = 0; i < n; i++) scanf("%lld", &a[i]);
	ll sum = 0;
	x.resize(m), w.resize(m); for (int i = 0; i < m; i++) scanf("%lld %lld", &x[i], &w[i]), x[i]--, sum += w[i];
	bool flag = true;
	for (int i = 0; i < n; i++) if (a[i] < sum) flag = false;
	if (flag) {
		puts("0");
		return 0;
	}
	ll left = 0;
	ll right = mod;
	while (right - left > 1) {
		ll mid = (left + right) >> 1;
		if (check(mid)) right = mid;
		else left = mid;
	}
	if (right == mod) right = -1;
	cout << right << endl;
	return 0;
	/*
		おまじないを使ったらscanfとprintf関連注意！！！！！！！！！！！！
	*/
}