#include <bits/stdc++.h>
#include <queue>
#include <vector>
using namespace std;
using ll = long long;
const int INF = 1e9 + 12;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<pair<int, int>> v(m);
for(auto &z : v) cin >> z.first >> z.second;
set<pair<int, int>> where[n];
for(auto z : v) {
where[z.first].insert(z);
}
vector<pair<int, int>> adj[n];
for(int i = 0; i < n; ++i) {
for(auto z : where[i]) {
if(z.second == 0) continue;
for(int j = 1; j < n; ++j) {
int nx = j * z.second + i;
if(nx >= n) break;
adj[i].push_back({nx, j});
if(where[nx].count({nx, z.second})) break;
}
for(int j = 1; j < n; ++j) {
int nx = -j * z.second + i;
if(nx < 0) break;
adj[i].push_back({nx, j});
if(where[nx].count({nx, z.second})) break;
}
}
}
// for(int i = 0; i < n; ++i) {
// cerr << i << ": ";
// for(auto z : adj[i]) {
// cerr << "{" << z.first << ", " << z.second << "}";
// cerr << ", ";
// }
// cerr << endl;
// }
auto calc = [&](int start, int end) {
vector<int> dist(n, INF);
dist[start] = 0;
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<>> pq;
pq.push({0, start});
while(!pq.empty()) {
auto tren = pq.top();
pq.pop();
if(dist[tren.second] != tren.first) continue;
for(auto e : adj[tren.second]) {
if(dist[e.first] > dist[tren.second] + e.second) {
dist[e.first] = dist[tren.second] + e.second;
pq.push({dist[e.first], e.first});
}
}
}
return dist[end];
};
int ans = calc(v[0].first, v[1].first);
cout << (ans >= INF ? -1 : ans) << '\n';
}
