#include <iostream>
#include <vector>
using namespace std;
using ll = long long;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N, M; cin >> N >> M;
  vector<int> a(N); for (int i = 0; i < N; i++) cin >> a[i];
  vector<int> x(M), w(M);
  int maxw = 0;
  for (int j = 0; j < M; j++) {
    cin >> x[j] >> w[j]; x[j]--;
    maxw = (maxw, w[j]);
  }

  int left = 0, right = maxw+1;
  while (left < right) {
    int c = (left + right) / 2;

    if (c == 0) {
      ll sumw = 0;
      for (int j = 0; j < M; j++) sumw += w[j];
      bool flag = false;
      for (int i = 0; i < N; i++) {
        if (a[i] <= sumw) {
          flag = true;
          break;
        }
      }
      if (flag)
        left = c + 1;
      else
        right = c;
      continue;
    }

    vector<ll> ft(N);
    for (int j = 0; j < M; j++) {
      ft[max(0, x[j] - w[j]/c)] += w[j];
      if (x[j] + w[j]/c + 1 < N)
        ft[x[j] + w[j]/c + 1] -= w[j];
    }
    for (int i = 1; i < N; i++) ft[i] += ft[i - 1];

    vector<ll> cd(N);
    for (int j = 0; j < M; j++) {
      cd[max(0, x[j] - w[j]/c)] += c;
      cd[x[j]] -= c;

      if (x[j] + 1 < N)
        cd[x[j] + 1] -= c;
      if (x[j] + w[j]/c + 1 < N)
        cd[x[j] + w[j]/c + 1] += c;
    }
    for (int i = 1; i < N; i++) cd[i] += cd[i-1];

    for (int j = 0; j < M; j++) {
      if (w[j]/c == 0) continue;

      if (x[j] - w[j]/c < 0)
        cd[0] -= x[j] ? (x[j] + 1) * c : 0;
      else
        cd[x[j] - w[j]/c] -= (w[j]/c + 1) * c;

      if (x[j] + w[j]/c + 1 < N)
        cd[x[j] + w[j]/c + 1] += w[j]/c * c;
    }
    for (int i = 1; i < N; i++) cd[i] += cd[i-1];

    bool flag = false;
    for (int i = 0; i < N; i++) {
      if (a[i] <= ft[i] + cd[i]) {
        flag = true;
        break;
      }
    }
    if (flag)
      left = c + 1;
    else
      right = c;
  }

  cout << (left < maxw+1 ? left : -1) << "\n";
}