#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
int n,m,s,e;
vector<pair<int,int> > adj[30001];
set<int> pos[30001];
int dist[30001];
bool vis[30001];
stack<int> st;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>n>>m;
for (int i=0;i<m;i++){
int b,p;
cin>>b>>p;
if (i==0) s=b;
if (i==1) e=b;
pos[p].insert(b);
}
for (int i=1;i<30001;i++){
for (auto j:pos[i]){
vis[j]=true;
st.push(j);
}
for (auto j:pos[i]){
int dis=1;
for (int k=j+i;k<n;k+=i){
adj[j].pb(mp(k,dis));
dis++;
if (vis[k]) break;
}
dis=1;
for (int k=j-i;k>=0;k-=i){
adj[j].pb(mp(k,dis));
dis++;
if (vis[k]) break;
}
}
while (!st.empty()){
vis[st.top()]=false;
st.pop();
}
}
for (int i=0;i<n;i++) dist[i]=1e9;
priority_queue<pair<int,int>,vector<pair<int,int> >,greater<pair<int,int> > >pq;
pq.push(mp(0,s));
dist[s]=0;
while (!pq.empty()){
pair<int,int> cur=pq.top();
pq.pop();
if (dist[cur.second]<cur.first) continue;
for (auto i:adj[cur.second]){
if (dist[i.first]>cur.first+i.second){
dist[i.first]=cur.first+i.second;
pq.push(mp(cur.first+i.second,i.first));
}
}
}
if (dist[e]==1e9) cout<<-1;
else cout<<dist[e];
}
