#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 = 5e6 + 20;
const int inf = 1e9 + 20;
int B[maxN];
int P[maxN];
vector<pair<int, bool>> adj[maxC];
int dist[maxC];
void just_do_it() {
int N, M;
cin >> N >> M;
const int lim = 0;
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) {
adj[cnt].emplace_back(cnt + 1, 1);
adj[cnt + 1].emplace_back(cnt, 1);
}
if (cur == B[i]) {
adj[B[i]].emplace_back(cnt, 0);
}
adj[cnt].emplace_back(cur, 0);
cnt++;
}
}
}
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();
q.pop_front();
for (auto p: adj[u]) {
int v = p.first;
int w = p.second;
if (dist[u] + w < dist[v]) {
dist[v] = dist[u] + w;
if (w == 0) {
q.push_front(v);
}
else {
q.push_back(v);
}
}
}
}
if (dist[B[1]] == inf) {
cout << -1;
}
else {
cout << dist[B[1]];
}
}
