#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  int n, k;
  std::cin >> n >> k;
  int64_t s1 = 0;
  int s2 = 0;
  for (int i = 0; i < k; ++i) {
    int a, b;
    std::cin >> a >> b;
    s1 += b;
    s2 = (s2 + 1ll * a * b) % n;
  }
  if (s1 > n) {
    std::cout << -1 << "\n";
  } else if (s1 < n) {
    std::cout << 1 << "\n";
  } else if (1ll * n * (n - 1) / 2 % n == s2) {
    std::cout << 1 << "\n";
  } else {
    std::cout << -1 << "\n";
  }
  return 0;
}
