#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
typedef vector<char> vc;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;
#define sep ' '
#define F first
#define S second
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define kill(x) {cout << x << endl;return;}
#define sz(x) int(x.size())
#define SP(x) setprecision(x)
#define mod(x) (1ll*x%M+M)%M
#define pq priority_queue
#define mid (l+r)/2
// #define mid2 (l+r+1)/2
#define pll pair<ll, ll>
#define REP(i, k) for (int i = 0 ; i < k ; i ++)
#define MP make_pair
const int M = 998244353;
const int N = 3e4+1;
const int T = 140;
int n, m;
vector<pair<int, int>> adj[N];
ll dis[N];
bool vis[N];
int b[N], p[N];
vi D[N];
bitset<T> Y[N];
void solve() {
// input in 0-index itself
cin >> n >> m;
for(int i = 0 ; i < m ; i ++) {
cin >> b[i] >> p[i];
D[b[i]].pb(p[i]);
}
for(int i = 0 ; i < n ; i ++) sort(D[i].begin(), D[i].end());
fill(dis, dis+n, n*n+1);
dis[b[0]] = 0;
priority_queue<pii, vector<pii>, greater<pii> > q;
q.push({0, b[0]});
while(!q.empty()) {
int u = q.top().S;
q.pop();
for(int i = 0 ; i < D[u].size() ; i ++) {
int s = D[u][i];
if (D[u][i] >= T) {
for(int neigh = u-s, w = 1; neigh >= 0 ; neigh -= s, w ++) {
if (dis[neigh] > dis[u] + w) {
dis[neigh] = dis[u] + w;
q.push({dis[neigh], neigh});
}
}
for(int neigh = u+s, w = 1; neigh < n ; neigh += s, w ++) {
if (dis[neigh] > dis[u] + w) {
dis[neigh] = dis[u] + w;
q.push({dis[neigh], neigh});
}
}
} else {
Y[u][s] = true;
for(int neigh = u-s, w = 1; neigh >= 0 && !Y[neigh][s] ; neigh -= s, w ++) {
if (dis[neigh] > dis[u] + w) {
dis[neigh] = dis[u] + w;
q.push({dis[neigh], neigh});
}
}
for(int neigh = u+s, w = 1; neigh < n && !Y[neigh][s] ; neigh += s, w ++) {
if (dis[neigh] > dis[u] + w) {
dis[neigh] = dis[u] + w;
q.push({dis[neigh], neigh});
}
}
}
}
}
cout << (dis[b[1]] == n*n+1 ? -1 : dis[b[1]]) << endl;
}
// check MAXN
int32_t main() {
fastIO;
solve();
return 0;
}
