#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long n, k;
  scanf("%lld%lld", &n, &k);
  long long coin = 0, sum = 0;
  for (long long i = 1; i <= k; i++) {
    long long a, b;
    scanf("%lld%lld", &a, &b);
    sum = (sum + a * b) % n;
    coin += b;
  }
  if (coin > n) {
    printf("-1");
    return 0;
  }
  if (coin < n) {
    printf("1");
    return 0;
  }
  if (sum == n * (n + 1) / 2 % n) {
    printf("1");
    return 0;
  }
  printf("-1");
  return 0;
}
