#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define fi first
#define se second
using namespace std;
const int MAXN = 3e4+10;
const int INF = 1e9+10;
typedef pair<int,int> pii;
typedef pair<int,pii> ipii;
int en, n, m;
int dis[MAXN], a[MAXN], b[MAXN];
vector <int> adj[MAXN];
priority_queue <pii,vector<pii>,greater<pii>> pq;
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n >> m;
for(int i=1; i<=m; i++){
cin >> a[i] >> b[i]; a[i]++;
adj[a[i]].pb(b[i]);
if(i==1) pq.push({0, a[i]});
if(i==2) en = a[i];
}
for(int i=1; i<=n; i++){
sort(adj[i].begin(), adj[i].end());
adj[i].resize(unique(adj[i].begin(), adj[i].end()) - adj[i].begin());
dis[i] = INF;
}
dis[a[1]] = 0;
while(!pq.empty()){
int dist = pq.top().fi, nw = pq.top().se; pq.pop();
if(nw==en){
cout << dis[nw] << '\n';
exit(0);
}
if(dist > dis[nw]) continue;
for(auto pow : adj[nw]){
int cnt = 1;
for(int i=nw+pow; i<=n; i+=pow){
if(dis[i] > dis[nw]+cnt){
dis[i] = dis[nw]+cnt;
pq.push({dis[i], i});
}
cnt++;
}
cnt = 1;
for(int i=nw-pow; i>=1; i-=pow){
if(dis[i] > dis[nw]+cnt){
dis[i] = dis[nw]+cnt;
pq.push({dis[i], i});
}
cnt++;
}
}
}
cout << "-1\n";
}
