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

  ll sow = 0;
  for (int i = 0; i < M; i++) sow += w[i];
  for (int i = 0; i < N; i++) a[i] = max(0LL, sow - a[i]);

  ll left = 0, right = 0, col = 0, cor = 0;
  for (int i = 1; i < N; i++) {
    cor += v[i];
    right += v[i] * i;
  }

  ll ans = 0;
  if (a[0] != 0) {
    if (right == 0) {
      cout << -1 << "\n";
      return 0;
    }
    ans = (a[0]+right-1) / right;
  }
  
  for (int i = 1; i < N; i++) {
    col += v[i-1];
    left += col;
    right -= cor;
    cor -= v[i];
    if (a[i] == 0) continue;
    if (left + right == 0) {
      cout << -1 << "\n";
      return 0;
    }
    ans = max(ans, (a[i]+left+right-1) / (left+right));
  }
  cout << ans << "\n";
}