#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define FOR(i,N) for(ll i = 0; i < N; i++)
#define all(x) (x).begin(), (x).end()
// #define F first
// #define S second
signed main() {
cin.tie(0); ios::sync_with_stdio(false);
int N, M;
cin >> N >> M;
int st, end;
vector<unordered_set<int>> B(N); // building -> power of doge inside
vector<vector<pair<int,int>>> adj(N);
FOR(i, M) {
int b, p;
cin >> b >> p;
B[b].insert(p);
if (i == 0) st = b;
if (i == 1) end = b;
}
// first 2 loops are O(M), 3 loops are O(NM)
FOR(u, N) {
for(int dog: B[u]) {
for(int v = u - dog; v >= 0; v -= dog) {
adj[u].push_back({v, (u - v) / dog});
if (B[v].find(dog) != B[v].end()) break;
}
for(int v = u + dog; v < N; v += dog) {
adj[u].push_back({v, (v - u) / dog});
if (B[v].find(dog) != B[v].end()) break;
}
}
}
priority_queue<pair<int,int>, vector<pair<int,int>>, greater<pair<int,int>>> pq;
vector<int> vis(N, 1e9);
pq.push({0, st});
while(pq.size()) {
auto [dist, u] = pq.top();
pq.pop();
if (u == end) {cout << dist; return 0;}
for(auto [v, j]: adj[u]) {
if (dist+j < vis[v]) {
pq.push({dist+j, v});
vis[v] = dist+j;
}
}
}
cout << -1;
}
