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);
        WriteLine(Bench(n, m, a, map));
    }
    static long Bench(int n, int m, int[] a, int[][] map)
    {
        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])
            {
                return -1;
            }
        }
        var ok = w.Max();
        if (a.Min() > w.Sum())
        {
            return 0;
        }
        var ng = 0L;
        while (ok - ng > 1)
        {
            var mid = (ok + ng) / 2;
            var imos = new long[n];
            var all = new long[n];
            foreach (var p in map)
            {
                var x = p[0] - 1;
                var wi = p[1];
                var pos = x - wi / mid;
                if (pos == x)
                {
                    all[x] += wi;
                    if (x + 1 < n) all[x + 1] -= wi;
                }
                else
                {
                    if (x == 0)
                    {
                        imos[0] -= mid;
                        all[0] += wi;
                    }
                    else if (pos <= 0)
                    {
                        imos[0] += mid;
                        all[0] += wi - mid * x;
                    }
                    else
                    {
                        imos[pos] += mid;
                        all[pos] += wi % mid;
                    }
                    if (x > 0)
                    {
                        imos[x] -= mid * 2;
                    }
                    if (wi % mid == 0)
                    {
                        pos = x + wi / mid;
                        if (pos < n) imos[pos] += mid;
                    }
                    else
                    {
                        pos = x + 1 + wi / mid;
                        if (pos < n)
                        {
                            imos[pos] += mid;
                            all[pos] += (mid - wi % mid) % mid;
                        }
                    }
                }
            }
            var flg = true;
            for (var i = 0; i < n; ++i)
            {
                if (i > 0)
                {
                    all[i] += all[i - 1] + imos[i - 1];
                    imos[i] += imos[i - 1];
                }
                if (all[i] >= a[i]) flg = false;
            }
            if (flg) ok = mid;
            else ng = mid;
        }
        return ok;
    }
}