#include <deque>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
cin.tie(0);
ios_base::sync_with_stdio(false);
int N, M;
cin >> N >> M;
vector<int> B(M), P(M);
vector<vector<int> > vmod(N), vx(N);
for (int i = 0; i < M; i++) {
cin >> B[i] >> P[i];
vx[B[i]].push_back(P[i]);
if (P[i] < N) vmod[P[i]].push_back(B[i] % P[i]);
}
for (int i = 0; i < N; i++) {
sort(vx[i].begin(), vx[i].end());
}
vector<vector<int> > g(N);
for (int i = 1; i < N; i++) {
sort(vmod[i].begin(), vmod[i].end());
vmod[i].erase(unique(vmod[i].begin(), vmod[i].end()), vmod[i].end());
for (int j : vmod[i]) {
int pre = -1;
for (int k = j; k < N; k += i) {
if (j != k) g[pre].push_back(g.size() * 2 + 1);
g.push_back(j == k ? vector<int>({ k * 2 }) : vector<int>({ k * 2, pre * 2 + 1 }));
pre = g.size() - 1;
if (binary_search(vx[k].begin(), vx[k].end(), i)) {
g[k].push_back(pre * 2);
}
}
}
}
vector<int> dist(g.size(), -1); dist[B[0]] = 0;
deque<int> que; que.push_back(B[0]);
while (!que.empty()) {
int u = que.front(); que.pop_front();
for (int e : g[u]) {
if (dist[e >> 1] == -1) {
dist[e >> 1] = dist[u] + (e & 1);
if (e & 1) que.push_back(e >> 1);
else que.push_front(e >> 1);
}
}
}
cout << dist[B[1]] << '\n';
return 0;
}
