#include <bits/stdc++.h>
using namespace std;
void just_do_it();
int main() {
#ifdef KAMIRULEZ
freopen("kamirulez.inp", "r", stdin);
freopen("kamirulez.out", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(0);
just_do_it();
return 0;
}
const int maxN = 3e4 + 20;
const int maxC = 2e6 + 20;
const int inf = 1e9 + 20;
int B[maxN];
int P[maxN];
vector<int> adj0[maxN];
vector<int> adj1[maxC];
int dist[maxC];
int pos[maxC];
bool flag[maxN];
int id[maxN];
void just_do_it() {
fill_n(pos, maxC, -1);
int N, M;
cin >> N >> M;
const int lim = sqrt(N);
int cnt = N;
for (int i = 0; i < M; i++) {
cin >> B[i] >> P[i];
if (P[i] > lim) {
int cur = B[i] % P[i];
int nxt = cur + P[i];
for (; cur < N; cur += P[i], nxt += P[i]) {
if (nxt < N) {
adj1[cnt].emplace_back(cnt + 1);
adj1[cnt + 1].emplace_back(cnt);
}
if (cur == B[i]) {
adj0[B[i]].emplace_back(cnt);
}
pos[cnt] = cur;
cnt++;
}
}
}
for (int d = 1; d <= lim; d++) {
for (int i = 0; i < N; i++) {
flag[i] = false;
}
bool found = false;
for (int i = 0; i < M; i++) {
if (P[i] == d) {
found = true;
break;
}
}
if (!found) {
continue;
}
for (int i = 0; i < d; i++) {
int cur = i;
int nxt = cur + d;
for (; cur < N; cur += d, nxt += d) {
if (nxt < N) {
adj1[cnt].emplace_back(cnt + 1);
adj1[cnt + 1].emplace_back(cnt);
}
id[cur] = cnt;
pos[cnt] = cur;
cnt++;
}
}
for (int i = 0; i < M; i++) {
if (P[i] == d && !flag[B[i]]) {
flag[B[i]] = true;
adj0[B[i]].emplace_back(id[B[i]]);
}
}
}
assert(cnt < maxC);
for (int i = 0; i < cnt; i++) {
dist[i] = inf;
}
dist[B[0]] = 0;
deque<int> q = {B[0]};
while (!q.empty()) {
int u = q.front();
if (u == B[1]) {
cout << dist[u];
return;
}
q.pop_front();
if (u < N) {
for (auto v: adj0[u]) {
if (dist[u] < dist[v]) {
dist[v] = dist[u];
q.push_front(v);
}
}
}
else if (pos[u] != -1) {
int v = pos[u];
if (dist[u] < dist[v]) {
dist[v] = dist[u];
q.push_front(v);
}
}
for (auto v: adj1[u]) {
if (dist[u] + 1 < dist[v]) {
dist[v] = dist[u] + 1;
q.push_back(v);
}
}
}
cout << -1;
}
