#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("Ofast,no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#define ll long long
#define fi first
#define se second
#define pb push_back
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int nax=3e4+5;
vector<int> adj[nax];
int dis[nax][300];
signed main(){
iostream::sync_with_stdio(false);
int n,m;
cin>>n>>m;
pair<int,int> fir;
pair<int,int> end;
for (int i = 0; i < m; ++i)
{
int x,y;
cin>>x>>y;
if(i==0) fir={x,y};
if(i==1) end={x,y};
adj[x].pb(y);
}
for (int i = 0; i < n; ++i)
{
for (int j = 0; j < 300; ++j)
{
dis[i][j]=1e9;
}
}
dis[fir.fi][0]=0;
priority_queue<tuple<int,int,int>,vector<tuple<int,int,int>>,greater<tuple<int,int,int>>> pq;
pq.push({0,fir.fi,0});
while(!pq.empty()){
auto [d , node , lst] = pq.top();
pq.pop();
if(node==end.fi){
cout <<d<<endl;
return 0;
}
if(dis[node][lst]<d) continue;
for(auto u:adj[node]){
if(u<300){
if(node-u>=0){
if(dis[node-u][u]>d+1){
dis[node-u][u]=d+1;
pq.push({d+1,node-u,u});
}
}if(node+u<n){
if(dis[node+u][u]>d+1){
dis[node+u][u]=d+1;
pq.push({d+1,node+u,u});
}
}
}else{
for (int i = 1; i < n; ++i)
{
if(u*i+node>=n) break;
if(dis[node+u*i][0]>d+i){
dis[node+u*i][0]=d+i;
pq.push({d+i,node+u*i,0});
}
}
for (int i = 1; i < n; ++i)
{
if(node-u*i<0) break;
if(dis[node-u*i][0]>d+i){
dis[node-u*i][0]=d+i;
pq.push({d+i,node-u*i,0});
}
}
}
}
if(lst!=0){
if(node-lst>=0){
if(dis[node-lst][lst]>d+1){
dis[node-lst][lst]=d+1;
pq.push({d+1,node-lst,lst});
}
}if(node+lst<n){
if(dis[node+lst][lst]>d+1){
dis[node+lst][lst]=d+1;
pq.push({d+1,node+lst,lst});
}
}
}
}
cout <<-1<<endl;
}
