#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, M;
    cin >> N >> M;
    vector<int> a(N), x(M), w(M);
    vector<vector<int>> pos_to_id(N);
    for(int i = 0; i < N; ++i)  cin >> a[i];
    for(int i = 0; i < M; ++i){
        cin >> x[i] >> w[i];
        --x[i];
        pos_to_id[x[i]].emplace_back(i);
    }

    int lb = -1, ub = 1000000;
    while(ub - lb > 1){
        int c = (lb + ub) / 2;
        if(c == 0){
            bool ok = true;
            ll sum = accumulate(begin(w), end(w), 0LL);
            for(int i = 0; i < N; ++i){
                if(sum >= a[i]) ok = false;
            }
            if(ok)  ub = c;
            else    lb = c;
            continue;
        }
        vector<vector<int>> start_id(N), end_id(N);
        for(int i = 0; i < M; ++i){
            int gap = w[i] / c;
            start_id[max(0, x[i] - gap)].emplace_back(i);
            end_id[min(N - 1, x[i] + gap)].emplace_back(i);
        }
        ll sum = 0;
        int left_cnt = 0, right_cnt = 0;
        bool ok = true;
        for(int i = 0; i < N; ++i){
            for(int& j : start_id[i]){
                sum += w[j] - (ll)(x[j] - i) * c;
                left_cnt += 1;
            }
            if(sum >= a[i]){
                ok = false;
                break;
            }
            for(int& j : pos_to_id[i]){
                left_cnt -= 1;
                right_cnt += 1;
            }
            for(int& j : end_id[i]){
                sum -= w[j] - (ll)(i - x[j]) * c;
                right_cnt -= 1;
            }
            sum += (ll)(left_cnt - right_cnt) * c;
        }

        if(ok)  ub = c;
        else    lb = c;
    }

    if(ub == 1000000)   ub = -1;
    cout << ub << endl;

    return 0;
}