#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include <debug.h>
#endif
#define st first
#define nd second
#define pb push_back
#define pf push_front
#define _pb pop_back
#define _pf pop_front
#define lb lower_bound
#define ub upper_bound
#define mtp make_tuple
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
typedef long long ll; typedef unsigned long long ull;
typedef double db; typedef long double ldb;
typedef pair<int, int> pi; typedef pair<ll, ll> pll;
typedef vector<int> vi; typedef vector<ll> vll; typedef vector<pi> vpi; typedef vector<pll> vpll;
typedef string str;
template<typename T> T gcd(T a, T b) { return (b == 0? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
#define FOR(i, l, r) for (int (i) = (l); (i) <= (r); ++(i))
#define FOS(i, r, l) for (int (i) = (r); (i) >= (l); --(i))
#define EACH(i, x) for (auto &(i) : (x))
#define WHILE while
#define file "TEST"
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
ll rand(ll l, ll r) { return uniform_int_distribution<ll>(l, r)(rd); }
/*
Tran The Bao
CTL - Da Lat
Cay ngay cay dem nhung deo duoc cong nhan
*/
struct Node {
int b, p, pos;
Node() {}
Node(int b, int p, int pos) : b(b), p(p), pos(pos) {}
};
const int oo = 1e9;
const int M = 7500001;
const int N = 30001;
int n, m, b[N], p[N], d[M], rs = oo, cnt1 = 0;
vi a[N];
pi stat[M];
deque<Node> q;
map<pi, int> cnt;
int getp(pi x) { return lb(stat + 1, stat + 1 + cnt1, x) - stat; }
signed main() {
ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
// freopen(file".inp", "r", stdin);
// freopen(file".out", "w", stdout);
cin >> n >> m;
FOR(i, 0, m - 1) {
cin >> b[i] >> p[i];
if (!cnt[{b[i] % p[i], p[i]}]) {
int bn = b[i], pn = p[i];
bn %= pn;
FOR(j, bn, n - 1) {
stat[++cnt1] = {j, pn};
j += pn - 1;
}
cnt[{b[i] % p[i], p[i]}] = 1;
}
a[b[i]].pb(p[i]);
}
sort(stat + 1, stat + 1 + cnt1);
FOR(i, 0, n - 1) sort(all(a[i]));
FOR(i, 0, M - 1) d[i] = oo;
int rpos = getp({b[0], p[0]});
d[rpos] = 0;
q.pb(Node(b[0], p[0], rpos));
// debug(stat);
WHILE(!q.empty()) {
Node cur = q.front();
q._pf();
int b = cur.b, p = cur.p, pos = cur.pos;
int du = d[pos];
if (b - p >= 0) {
int posv = getp({b - p, p});
if (d[posv] > du + 1) {
d[posv] = du + 1;
q.pb(Node(b - p, p, posv));
}
}
if (b + p < n) {
int posv = getp({b + p, p});
if (d[posv] > du + 1) {
d[posv] = du + 1;
q.pb(Node(b + p, p, posv));
}
}
if (!a[b].empty()) {
if (a[b].back() < p) {
int p1 = a[b][0];
int posv = getp({b, p1});
if (d[posv] > du) {
d[posv] = du;
q.pf(Node(b, p1, posv));
}
}
else {
int pos = lb(all(a[b]), p) - a[b].begin();
if (a[b][pos] != p) {
int p1 = a[b][0];
int posv = getp({b, p1});
if (d[posv] > du) {
d[posv] = du;
q.pf(Node(b, p1, posv));
}
}
else {
if (pos > 0) {
int p1 = a[b][pos - 1];
int posv = getp({b, p1});
if (d[posv] > du) {
d[posv] = du;
q.pf(Node(b, p1, posv));
}
}
if (pos < sz(a[b]) - 1) {
int p1 = a[b][pos + 1];
int posv = getp({b, p1});
if (d[posv] > du) {
d[posv] = du;
q.pf(Node(b, p1, posv));
}
}
}
}
}
}
FOR(i, 1, N - 1) {
if (stat[cnt1] < pi(b[1], i)) continue;
int pos = getp({b[1], i});
if (stat[pos] != pi(b[1], i)) continue;
rs = min(rs, d[pos]);
}
cout << (rs < oo? rs : -1);
return 0;
}
/*
5 3
0 2
1 1
4 1
*/
