#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <set>
#include <random>
#include <iomanip>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)

template<class T>
using vi = vector<T>;

template<class T>
using vii = vector<vi<T>>;

template<class T>
using viii = vector<vii<T>>;

void chmax(ll & x, ll y) { x = max(x, y); }


ll gcd(ll x, ll y) {
    if (y == 0) {
        return x;
    }
    return gcd(y, x % y);
    // return y ? gcd(y, x%y) : x;
}

//ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

ll lcm(ll x, ll y) {
    return x / gcd(x, y) * y;
}

ll extgcd(ll x, ll y, ll& a, ll& b) {
    if (y == 0) {
        a = 1LL, b = 0LL;
        return x;
    }
    ll g = extgcd(y, x % y, a, b);
    ll ta = a, tb = b;
    a = tb;
    b = ta - x / y * tb;
    if (a < 0LL) a += y / g, b -= x / g; //when you wanna a >= 0 
    return g;
}

struct status {
    ll g, lcm, cr;
};

//m(mod x), n(mod y)
status crt(ll x, ll y, ll m, ll n) {
    status res;
    ll a, b;
    res.g = extgcd(x, y, a, b); //assume non-zero
    //g = xa + yb

    res.lcm = x / res.g * y;
    if ((m - n) % res.g) res.cr = -1;
    else {//xa'+m=-yb'+n ⇔ xa'+yb'=n-m=kg ⇔ xak+ybk = kg
        res.cr = (n - m) / res.g * a % (y / res.g); //係数の桁数を先に落とす
        res.cr = res.cr * x + m;
        res.cr = (res.cr % res.lcm + res.lcm) % res.lcm;
    }
    return res;
}
int main()
{
    int n, m;
    cin >> n >> m;

    vi<ll> b(m), c(m);
    rep(i, m) cin >> b[i] >> c[i];

    status ans = {0, 0, 0};
    rep(i, m) {
        c[i] -= c[i] / b[i] * b[i];
        if (c[i] < 0) c[i] += b[i];

        if (ans.g == 0) {
            ans.g = b[i];
            ans.lcm = b[i];
            ans.cr = c[i];
            continue;
        }

        ans = crt(ans.lcm, b[i], ans.cr, c[i]);
        if (ans.cr < 0 || ans.cr > n) {
            rep(idx, 100) {
                bool ok = true;
                rep(j, m) {
                    if ((idx - c[j]) % b[j]) {
                        ok = false;
                        break;
                    }
                }
                if (ok) {
                    cout << idx << endl;
                    return 0;
                }
            }

            cout << "NaN" << endl;
            return 0;
        }
    }
    cout << ans.cr << endl;
	return 0;
}