/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.06.20 19:28:53
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int n,m;cin >> n >> m;
    vector<int> a(n);
    ll sum = 0;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    vector<int> b(n);
    for (int i = 0; i < m; i++) {
        int x,w;cin >> x >> w;
        x--;
        sum += w;
        b[0] += min(x,w);
        if (n > 1) b[1] -= min(x,w);
        if (x <= w && n > 1) {
            b[1] -= 1;
        }
        else if (x - w + 1 >= 0 && x - w + 1 < n) {
            b[x-w+1] -= 1;
        }
        if (x + 1 < n) b[x+1] += 2;
        if (x + w + 1 < n) b[x + w + 1] -= 1;
    }
    for (int i = 0; i < n - 1; i++) {
        b[i + 1] += b[i];
    }
    for (int i = 0; i < n - 1; i++) {
        b[i + 1] += b[i];
    }
    ll l = -1, r = 10000000001;
    while(r - l > 1) {
        ll mid = l + (r - l) / 2;
        bool ok = true;
        for (int i = 0; i < n; i++) {
            if (a[i] <= sum - mid * b[i]) {
                ok = false;
                break;
            }
        }
        if (ok) {
            r = mid;
        }
        else {
            l = mid;
        }
    }
    if (r == 10000000001) {
        cout << -1 << endl;
    }
    else {
        cout << r << endl;
    }
    return 0;
}