#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define sky first
#define pow second
const int nax = 30005;
const int INF = 1e9 + 5;
vector<pii> adj[nax];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<pii> a(m);
for(auto& i : a) {
cin >> i.sky >> i.pow;
}
set<pii> used;
set<pii> all(a.begin(), a.end());
for(auto i : a) {
if (used.count(i)) continue;
used.insert(i);
int idx = i.sky + i.pow;
for(int cost = 1;;cost++) {
if (idx >= n) break;
adj[i.sky].push_back({idx, cost});
if (all.count({idx, i.pow})) break;
// cout << i.sky << "->" << idx << " " << cost << "\n";
idx += i.pow;
}
idx = i.sky - i.pow;
for(int cost = 1;;cost++) {
if (idx < 0) break;
adj[i.sky].push_back({idx, cost});
if (all.count({idx, i.pow})) break;
// cout << i.sky << "->" << idx << " " << cost << "\n";
idx -= i.pow;
}
}
vector<int> dist(n, INF);
priority_queue<pii, vector<pii>, greater<pii>> q;
q.push({a[0].sky, 0});
dist[a[0].sky] = 0;
while(!q.empty()) {
pii v = q.top();
q.pop();
// cout << v.sky << " " << v.pow << "\n";
if (dist[v.sky] < v.pow) continue;
if (v.sky == a[1].sky) {
cout << dist[a[1].sky] << "\n";
exit(0);
}
for(pii u : adj[v.sky]) {
int cost = u.pow + v.pow;
if (dist[u.sky] > cost) {
dist[u.sky] = cost;
q.push({u.sky, dist[u.sky]});
}
}
}
cout << (dist[a[1].sky] == INF ? -1 : dist[a[1].sky]) << "\n";
return 0;
}
