using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics.X86;
using System.Runtime.Intrinsics.Arm;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m) = (c[0], c[1]);
        var a = NList;
        var map = NArr(m);
        var w = new long[n];
        foreach (var p in map)
        {
            w[p[0] - 1] += p[1];
        }
        for (var i = 0; i < n; ++i)
        {
            if (a[i] < w[i])
            {
                WriteLine(-1);
                return;
            }
        }
        var ok = w.Max();
        if (a.Min() >= w.Sum())
        {
            WriteLine(0);
            return;
        }
        var ng = 0L;
        while (ok - ng > 1)
        {
            var mid = (ok + ng) / 2;
            var imos = new long[n];
            var all = new long[n];
            for (var i = 0; i < n; ++i) if (w[i] > 0)
                {
                    var pos = i - w[i] / mid;
                    if (i == 0)
                    {
                        all[0] += w[i] - mid;
                        imos[0] += mid;
                    }
                    else if (pos <= 0)
                    {
                        all[0] += w[i] - mid * (i + 1);
                        imos[0] += mid;
                    }
                    else
                    {
                        all[pos] += w[i] % mid;
                        imos[pos + 1] += mid;
                    }
                    pos = i + 1;
                    if (pos < n) imos[pos] -= mid * 2;
                    pos += w[i] / mid;
                    if (pos < n)
                    {
                        all[pos] += (mid - w[i] % mid) % mid;
                    }
                    ++pos;
                    if (pos < n) imos[pos] += mid;
                }
            var flg = true;
            for (var i = 0; i < n; ++i)
            {
                if (i > 0)
                {
                    imos[i] += imos[i - 1];
                    all[i] += all[i - 1];
                }
                all[i] += imos[i];
                if (all[i] > a[i]) flg = false;
            }
            if (flg) ok = mid;
            else ng = mid;
        }
        WriteLine(ok);
    }
}