#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007;
// const int MOD = 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
const int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

template <typename Abelian>
struct BITRangeAdd {
  BITRangeAdd(int n_, const Abelian UNITY = 0) : n(n_), UNITY(UNITY) {
    ++n;
    dat_const.assign(n, UNITY);
    dat_linear.assign(n, UNITY);
  }

  void add(int left, int right, Abelian val) {
    if (right < ++left) return;
    for (int i = left; i < n; i += i & -i) {
      dat_const[i] -= val * (left - 1);
      dat_linear[i] += val;
    }
    for (int i = right + 1; i < n; i += i & -i) {
      dat_const[i] += val * right;
      dat_linear[i] -= val;
    }
  }

  Abelian sum(int idx) {
    Abelian res = UNITY;
    for (int i = idx; i > 0; i -= i & -i) res += dat_linear[i];
    res *= idx;
    for (int i = idx; i > 0; i -= i & -i) res += dat_const[i];
    return res;
  }

  Abelian sum(int left, int right) {
    if (right <= left) return UNITY;
    return sum(right) - sum(left);
  }

  Abelian operator[](const int idx) { return sum(idx, idx + 1); }

private:
  int n;
  const Abelian UNITY;
  vector<Abelian> dat_const, dat_linear;
};

int main() {
  int n, m; cin >> n >> m;
  vector<int> a(n); REP(i, n) cin >> a[i];
  vector<int> x(m), w(m); REP(i, m) cin >> x[i] >> w[i], --x[i];
  vector<ll> bench(n, 0);
  REP(i, m) bench[x[i]] += w[i];
  REP(i, n) {
    if (bench[i] >= a[i]) {
      cout << "-1\n";
      return 0;
    }
  }
  if (*min_element(ALL(a)) > accumulate(ALL(w), 0LL)) {
    cout << "0\n";
    return 0;
  }
  ll lb = 0, ub = 100000;
  while (ub - lb > 1) {
    ll mid = (lb + ub) >> 1;
    BITRangeAdd<ll> ci(n), left(n), right(n);
    REP(i, m) {
      int ran = w[i] / mid;
      int l = min(x[i], ran), r = min(n - 1 - x[i], ran);
      ci.add(x[i] - l, x[i] + r + 1, w[i]);
      left.add(x[i] - l, x[i], -1);
      if (x[i] - l > 0) left.add(x[i] - l - 1, x[i] - l, l);
      right.add(x[i] + 1, x[i] + r + 1, -1);
      if (x[i] + r + 1 < n) right.add(x[i] + r + 1, x[i] + r + 2, r);
    }
    vector<ll> L(n), R(n);
    REP(i, n) L[i] = left[i];
    for (int i = n - 2; i >= 0; --i) L[i] += L[i + 1];
    REP(i, n) R[i] = right[i];
    FOR(i, 1, n) R[i] += R[i - 1];
    bool ok = true;
    // cout << mid << ":\n";
    // REP(i, n) cout << ci[i] << " \n"[i + 1 == n];
    // REP(i, n) cout << L[i] << " \n"[i + 1 == n];
    // REP(i, n) cout << R[i] << " \n"[i + 1 == n];
    REP(i, n) {
      ll load = ci[i] + (L[i] + R[i]) * mid;
      // cout << load << " \n"[i + 1 == n];
      ok &= load < a[i];
    }
    (ok ? ub : lb) = mid;
  }
  cout << ub << '\n';
  return 0;
}