#include <bits/stdc++.h>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
// const ll mod = 998244353;
const ll INF = mod * mod;
const int INF_N = 1e+9;
typedef pair<int, int> P;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acos(-1.0);
//typedef vector<vector<ll>> mat;
typedef vector<int> vec;

//繰り返し二乗法
ll mod_pow(ll a, ll n, ll m) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * a%m;
		a = a * a%m; n >>= 1;
	}
	return res;
}

struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n%mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint &a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint &a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint &a, modint b) { a.n = ((ll)a.n*b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
	if (n == 0)return modint(1);
	modint res = (a*a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

//逆元(Eucledean algorithm)
ll inv(ll a, ll p) {
	return (a == 1 ? 1 : (1 - p * inv(p%a, a)) / a + p);
}
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 18;
modint fact[max_n], factinv[max_n];
void init_f() {
	fact[0] = modint(1);
	for (int i = 0; i < max_n - 1; i++) {
		fact[i + 1] = fact[i] * modint(i + 1);
	}
	factinv[max_n - 1] = modint(1) / fact[max_n - 1];
	for (int i = max_n - 2; i >= 0; i--) {
		factinv[i] = factinv[i + 1] * modint(i + 1);
	}
}
modint comb(int a, int b) {
	if (a < 0 || b < 0 || a < b)return 0;
	return fact[a] * factinv[b] * factinv[a - b];
}
using mP = pair<modint, modint>;

int dx[4] = { 0,1,0,-1 };
int dy[4] = { 1,0,-1,0 };


const ll ma = 1e09 + 1;

void solve() {
    int n, m; cin >> n >> m;
    vector<ll> a(n); rep(i, n) cin >> a[i];
    ll l=0, r=ma;
    vector<ll> x(m), w(m), ww(n, 0);
    ll sum = 0;
    rep(i, m){
        cin >> x[i] >> w[i];
        x[i]--;
        ww[x[i]] = w[i];
        sum += w[i];
    }

    {
        bool f = true;
        rep(i, n) if(sum >= a[i]) f = false;
        if(f){
            cout << 0 << endl;
            return;
        }
    }

    vector<ll> res1(n, 0), res2(n, 0);
    while(r-l > 1){
        ll mid = (r+l)/2;

        vector<ll> lv(n, 0), rv(n, 0);
        rep(i, m){
            ll id = (w[i]+mid-1)/mid;
            lv[x[i]] += 1;
            rv[x[i]] += 1;
            if(x[i]+id < n){
                lv[x[i]+id] += -1;
            }
            if(x[i]-id >= 0){
                rv[x[i]-id] += -1;
            }
        }

        rep(i, n-1){
            lv[i+1] += lv[i];
        }
        per(i, n-1){
            rv[i] += rv[i+1];
        }

        ll cnt = 0;
        rep(i, n){
            if(i){
                res1[i] = max(0LL, res1[i-1] - cnt*mid) + ww[i];
                cnt = lv[i];
            }else{
                res1[i] = ww[i];
                cnt = 1;
            }
        }

        cnt = 0;
        per(i, n){
            if(i < n-1){
                res2[i] = max(0LL, res2[i+1] - cnt*mid) + ww[i];
                cnt = rv[i];
            }else{
                res2[i] = ww[i];
                cnt = 1;
            }
        }

        vector<ll> res(n);
        rep(i, n) res[i] = res1[i] + res2[i] - ww[i];
        bool f = true;
        rep(i, n) if(res[i] >= a[i]) f = false;

        if(f) r = mid;
        else l = mid;
    }

    if(r == ma) cout << -1 << endl;
    else cout << r << endl;
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  //cout << fixed << setprecision(10);
  //init_f();
  //init();
  //int t; cin >> t; rep(i, t)solve();
  solve();
//   stop
    return 0;
}