/*************************************
* author: marvinthang *
* created: 03.02.2023 14:54:32 *
*************************************/
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define left ___left
#define right ___right
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)
#define MASK(i) (1LL << (i))
#define BIT(x, i) ((x) >> (i) & 1)
#define __builtin_popcount __builtin_popcountll
#define ALL(v) (v).begin(), (v).end()
#define REP(i, n) for (int i = 0, _n = (n); i < _n; ++i)
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; ++i)
#define FORD(i, b, a) for (int i = (b), _a = (a); i >= _a; --i)
#define scan_op(...) istream & operator >> (istream &in, __VA_ARGS__ &u)
#define print_op(...) ostream & operator << (ostream &out, const __VA_ARGS__ &u)
#ifdef LOCAL
#include "debug.h"
#else
#define file(name) if (fopen(name".inp", "r")) { freopen(name".inp", "r", stdin); freopen(name".out", "w", stdout); }
#define DB(...) 23
#define db(...) 23
#define debug(...) 23
#endif
template <class T> scan_op(vector <T>) { for (size_t i = 0; i < u.size(); ++i) in >> u[i]; return in; }
template <class U, class V> scan_op(pair <U, V>) { return in >> u.fi >> u.se; }
template <class U, class V> print_op(pair <U, V>) { return out << '(' << u.first << ", " << u.second << ')'; }
template <class Con, class = decltype(begin(declval<Con>()))> typename enable_if <!is_same<Con, string>::value, ostream&>::type operator << (ostream &out, const Con &con) { out << '{'; for (__typeof(con.begin()) it = con.begin(); it != con.end(); ++it) out << (it == con.begin() ? "" : ", ") << *it; return out << '}'; }
template <size_t i, class T> ostream & print_tuple_utils(ostream &out, const T &tup) { if constexpr(i == tuple_size<T>::value) return out << ")"; else return print_tuple_utils<i + 1, T>(out << (i ? ", " : "(") << get<i>(tup), tup); }
template <class ...U> print_op(tuple<U...>) { return print_tuple_utils<0, tuple<U...>>(out, u); }
template <class A, class B> bool minimize(A &a, B b) { if (a > b) { a = b; return true; } return false; }
template <class A, class B> bool maximize(A &a, B b) { if (a < b) { a = b; return true; } return false; }
// end of template
const int MAX = 5e4 + 4;
const int BLOCK_SIZE = 150;
int N, M, B[MAX], P[MAX];
void init(void) {
cin >> N >> M;
REP(i, M) cin >> B[i] >> P[i];
}
bool exist[BLOCK_SIZE][BLOCK_SIZE];
vector <pair <int, int>> adj[MAX];
pair <int, int> rev_id[MAX * BLOCK_SIZE];
int dist[MAX * BLOCK_SIZE + MAX];
int id[BLOCK_SIZE][MAX];
bool inQueue[MAX * BLOCK_SIZE + MAX];
void process(void) {
REP(i, M) if (P[i] < BLOCK_SIZE) exist[P[i]][B[i] % P[i]] = true;
int nn = N;
FOR(i, 1, BLOCK_SIZE - 1) REP(j, i) if (exist[i][j]) for (int k = j; k < N; k += i) {
id[i][k] = nn++;
rev_id[id[i][k] - N] = make_pair(i, k);
}
REP(i, M) {
if (P[i] >= BLOCK_SIZE) {
for (int j = 1, k = B[i]; (k -= P[i]) >= 0; ++j)
adj[B[i]].push_back(make_pair(j, k));
for (int j = 1; B[i] + j * P[i] < N; ++j)
adj[B[i]].push_back(make_pair(j, B[i] + j * P[i]));
} else adj[B[i]].push_back(make_pair(0, id[P[i]][B[i]]));
}
queue <int> q;
memset(dist, 0x3f, sizeof(dist));
dist[B[0]] = 0;
q.push(B[0]);
inQueue[B[0]] = true;
while (!q.empty()) {
int u = q.front(); q.pop();
inQueue[u] = false;
if (u >= N) {
auto &[p, b] = rev_id[u - N];
if (minimize(dist[b], dist[u]) && !inQueue[b]) {
q.push(b);
inQueue[b] = true;
}
if (b >= p && minimize(dist[id[p][b - p]], dist[u] + 1) && !inQueue[id[p][b - p]]) {
q.push(id[p][b - p]);
inQueue[id[p][b - p]] = true;
}
if (b + p < N && minimize(dist[id[p][b + p]], dist[u] + 1) && !inQueue[id[p][b + p]]) {
q.push(id[p][b + p]);
inQueue[id[p][b + p]] = true;
}
} else {
for (auto &[w, v]: adj[u]) if (minimize(dist[v], dist[u] + w) && !inQueue[v]) {
q.push(v);
inQueue[v] = true;
}
}
}
cout << (dist[B[1]] < 1e9 ? dist[B[1]] : -1) << '\n';
}
int main(void) {
ios_base::sync_with_stdio(false); cin.tie(nullptr); // cout.tie(nullptr);
file("skyscraper");
init();
process();
// cerr << "Time elapsed: " << TIME << " s.\n";
return (0^0);
}
