#include <bits/stdc++.h>
using std::abs;
using std::array;
using std::cerr;
using std::cin;
using std::cout;
using std::generate;
using std::get;
using std::make_pair;
using std::make_tuple;
using std::map;
using std::max;
using std::max_element;
using std::min;
using std::min_element;
using std::pair;
using std::reverse;
using std::set;
using std::sort;
using std::string;
using std::swap;
using std::tuple;
using std::unique;
using std::vector;
template <typename T>
T input() {
  T res;
  cin >> res;
  {};
  return res;
}
template <typename IT>
void input_seq(IT b, IT e) {
  std::generate(b, e,
                input<typename std::remove_reference<decltype(*b)>::type>);
}
int main() {
  std::iostream::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n = input<int>();
  int k = input<int>();
  int64_t total = 0;
  int remainder = 0;
  for (int i = 0; i < k; ++i) {
    int a, b;
    cin >> a >> b;
    a -= 1;
    total += b;
    remainder = (remainder + a * (int64_t)b) % n;
  }
  int want = (n % 2 == 1 ? 0 : n / 2);
  if (total < n or (total == n and remainder == want))
    cout << "1\n";
  else
    cout << "-1\n";
  return 0;
}
