#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  n++;
  vector<P> bc(m);
  for(auto& [b, c]: bc) {
    cin >> b >> c;
    c = (c % b + b) % b;
  }
  sort(all(bc));
  bc.erase(unique(all(bc)), bc.end());
  m = bc.size();
  rep(i, m - 1) if (bc[i].first == bc[i+1].first) {
    if (bc[i].second != bc[i+1].second) {
      cout << "NaN\n";
      return 0;
    }
  }
  VI cnt(n);
  for(auto [b, c]: bc) {
    for(int i = c; i < n; i += b) cnt[i]++;
  }
  rep(i, n) if (cnt[i] == m) {
    cout << i << '\n';
    return 0;
  }
  cout << "NaN\n";
}