#include <bits/stdc++.h>
#define ALL(v) std::begin(v), std::end(v)
#define ALLR(v) std::rbegin(v), std::rend(v)
using ll = std::int64_t;
using ull = std::uint64_t;
using pll = std::pair<ll, ll>;
using tll = std::tuple<ll, ll, ll>;
template <typename T> using vec = std::vector<T>;
template <typename T> using vvec = vec<vec<T>>;
template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return std::min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return std::max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return std::max(low, std::min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { return t = clamp(t, low, high); }
namespace init__ { struct InitIO { InitIO() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << std::fixed << std::setprecision(30); } } init_io; }
#define mv_rec make_v(init, tail...)
template <typename T> T make_v(T init) { return init; }
template <typename T, typename... Tail> auto make_v(T init, size_t s, Tail... tail) { return vec<decltype(mv_rec)>(s, mv_rec); }
#undef mv_rec
namespace ios { char line = '\n'; }
template <typename T> std::ostream& operator <<(std::ostream &os, const vec<T> &v) { for (ll i = 0; i < v.size(); i++) os << v[i] << " \n"[i + 1 == v.size()]; return os; }

struct Solver {
    ll n, m;
    vec<ll> av;
    vec<pll> xw;
    vec<ll> imos1, imos2;
    constexpr static std::size_t SIZE = 2e5 + 10;

    Solver(ll n, ll m) : n(n), m(m), av(n), xw(m), imos1(2 * SIZE), imos2(2 * SIZE) {
        for (auto &&e : av) std::cin >> e;
        for (auto &&e : xw) {
            ll x, w;
            std::cin >> x >> w;
            e = pll(x - 1, w);
        }
    }

    bool check(ll c) {
        fill(ALL(imos1), 0);
        fill(ALL(imos2), 0);

        auto calc_len = [&](ll w) {
            return (c == 0 ? n + 5 : w / c);
        };

        for (ll j = 0; j < m; j++) {
            ll x, w;
            std::tie(x, w) = xw[j];
            ll len = calc_len(w);
            imos1[x + 1] -= c;
            imos1[x + 1 + len] += c;
            imos2[x - 1 + SIZE] -= c;
            imos2[x - 1 - len + SIZE] += c;
        }

        auto fold_imos = [&] {
            for (ll i = 0; i + 1 < imos1.size(); i++) imos1[i + 1] += imos1[i];
            for (ll i = imos2.size() - 1; 0 <= i - 1; i--) imos2[i - 1] += imos2[i];
        };

        fold_imos();

        for (ll j = 0; j < m; j++) {
            ll x, w;
            std::tie(x, w) = xw[j];
            ll len = calc_len(w);
            ll sumv = c * len;
            imos1[x] += w;
            imos1[x + 1 + len] += -w + sumv;
            imos2[x + SIZE] += w;
            imos2[x - 1 - len + SIZE] += -w + sumv;
        }
        
        fold_imos();
        for (ll j = 0; j < m; j++) {
            ll x, w;
            std::tie(x, w) = xw[j];
            imos2[x + SIZE] -= w;
        }

        for (ll i = 0; i < n; i++) {
            ll a = av[i];
            a -= imos1[i];
            a -= imos2[i + SIZE];
            if (a <= 0) return false;
        }

        return true;
    }

    ll solve() {
        ll init = 1e5 + 10;
        ll ok = init, ng = -1;
        while (ok - ng > 1) {
            ll mid = (ok + ng) / 2;
            (check(mid) ? ok : ng) = mid;
        }

        return (ok == init ? -1 : ok);
    }
};

int main() {
    ll n, m;
    std::cin >> n >> m;
    Solver solver(n, m);
    std::cout << solver.solve() << ios::line;
    return 0;
}