#include <bits/stdc++.h>
using namespace std;
#define int long long
#define inf 1e18
const int MXN = 3e4 + 5;
int b[MXN], p[MXN], dist[MXN];
vector<int> adj[MXN];
map<int, int> mp[MXN];
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
for (int i = 0; i < n; i++) dist[i] = inf;
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
adj[b[i]].push_back(p[i]);
}
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
dist[b[0]] = 0;
pq.push({0, b[0]});
while (!pq.empty()) {
auto [k, u] = pq.top();
pq.pop();
if (k > dist[u]) continue;
for (int v : adj[u]) {
// if (mp[v].find(u % v) != mp[v].end()) continue;
// mp[v][u % v];
for (int i = u - v; i >= 0; i -= v) {
if (dist[i] > dist[u] + ((u - i) / v)) {
dist[i] = dist[u] + ((u - i) / v);
pq.push({dist[i], i});
}
}
for (int i = u + v; i < n; i += v) {
if (dist[i] > dist[u] + ((i - u) / v)) {
dist[i] = dist[u] + ((i - u) / v);
pq.push({dist[i], i});
}
}
}
}
cout << (dist[b[1]] >= inf ? -1 : dist[b[1]]) << '\n';
}
