/**
 *    author:  otera    
**/
#include<iostream>
#include<string>
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<deque>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

#define int long long
typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const int inf=1e9+7;
const ll INF=1LL<<60 ;
const ll mod=1e9+7 ;
#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++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(),c.end()
#define pb push_back
#define debug(x)  cerr << #x << " = " << (x) << endl;
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; }

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

bool check(int c) {
    vector<int> b(n + 1, 0);
    rep(j, n) {
        int cnt = x[j] / c;
        if(x[j] - cnt >= 0) b[x[j] - cnt] += w[j] % c;
        if(x[j] - cnt >= 0) {
            b[x[j] - cnt + 1] += c - w[j] % c;
        } else {
            b[0] += c;
        }
        if(x[j] + 1 < n) {
            b[x[j] + 1] += -2 * c;
        }
        if(x[j] + cnt + 1 < n) {
            b[x[j] + cnt + 1] += c - w[j] % c;
        }
        if(x[j] + cnt + 2 < n) {
            b[x[j] + cnt + 2] += w[j] % c;
        }
    }

    for(int j = 1; j < n; ++j) {
        b[j] += b[j - 1];
    }
    rep(i, n) {
        if(a[i] < b[i]) {
            return false;
        }
    }
    return true;
}

void solve() {
	cin >> n >> m;
    a.resize(n);
    rep(i, n) {
        cin >> a[i];
    }
    x.resize(m);
    w.resize(m);
    rep(i, m) {
        cin >> x[i] >> w[i];
        --x[i];
    }
    int ok = inf, ng = -1;
    if(!check(ok)) {
        cout << -1 << endl;
        return;
    }
    while(ok - ng > 1) {
        int mid = (ok + ng) / 2;
        if(check(mid)) {
            ok = mid;
        } else {
            ng = mid;
        }
    }
    cout << ok << endl;
}

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