#include <bits/extc++.h>

int main() {
    using namespace std;
    unsigned long N, M;
    cin >> N >> M;
    vector<pair<unsigned long, unsigned long>> v(M);
    for(auto&& [B, C] : v){
        cin >> B;
        long c;
        cin >> c;
        C = (c % static_cast<long>(B) + B) % B;
    }
    sort(begin(v), end(v));
    for(unsigned long i{1}; i < M; ++i)if(v[i - 1].first == v[i].first && v[i - 1].second != v[i].second){
        cout << "NaN" << endl;
        return 0;
    }
    v.erase(unique(begin(v), end(v)), end(v));
    vector<unsigned long> possible(N + 1);
    iota(begin(possible), end(possible), 0UL);
    for(const auto& [B, C] : v){
        possible.erase(remove_if(begin(possible), end(possible), [&B, &C](auto&& i){return i % B != C;}), end(possible));
        if(empty(possible)){
            cout << "NaN" << endl;
            return 0;
        }
    }
    cout << possible[0] << endl;
    return 0;
}