#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define fbo find_by_order
#define ook order_of_key
typedef long long ll;
typedef pair<int,int> ii;
typedef vector<ll> vi;
typedef long double ld;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
typedef set<int>::iterator sit;
typedef map<int,int>::iterator mit;
typedef vector<int>::iterator vit;
const int INF = int(1e9);
const int C = 62;
const int N = 30001*C;
vector<ii> adj[N+30001];
int dist[N+30001];
void addedge(int u, int v, int w)
{
adj[u].pb(mp(v,w));
}
int getid(int u, int v)
{
return (u*C+v);
}
set<int> S[30001];
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int n,m; cin>>n>>m;
int s,e;
for(int i=0;i<m;i++)
{
int x,y;
cin>>x>>y;
if(i==0) s=x;
if(i==1) e=x;
S[x].insert(y);
}
for(int i=0;i<n;i++)
{
for(sit it = S[i].begin(); it != S[i].end(); it++)
{
int y=(*it);
if(y<C)
{
addedge(i+N,getid(i,y),0);
/*
if(i-y>=0)
{
addedge(getid(i,y),getid(i-y,y),1);
//addedge(getid(i,y),i-y+N,1);
}
if(i+y<n)
{
addedge(getid(i,y),getid(i+y,y),1);
//addedge(getid(i,y),i+y+N,1);
}
*/
}
else
{
int cnt=0;
for(int j=i+y;j<n;j+=y)
{
addedge(i+N,j+N,++cnt);
}
cnt=0;
for(int j=i-y;j>=0;j-=y)
{
addedge(i+N,j+N,++cnt);
}
}
}
}
for(int i=0;i<n;i++)
{
for(int j=1;j<C;j++)
{
if(i-j>=0)
{
addedge(getid(i,j),i-j+N,1);
addedge(getid(i,j),getid(i-j,j),1);
}
if(i+j<n)
{
addedge(getid(i,j),getid(i+j,j),1);
addedge(getid(i,j),i+j+N,1);
}
}
}
e+=N;
s+=N;
priority_queue<ii,vector<ii>,greater<ii> > pq;
for(int i=0;i<N+30001;i++) dist[i]=INF;
pq.push(mp(0,s));
dist[s]=0;
while(!pq.empty())
{
int d = pq.top().fi; int u = pq.top().se; pq.pop();
//cerr<<d<<' '<<u<<'\n';
for(int i=0;i<adj[u].size();i++)
{
int v=adj[u][i].fi; int w=adj[u][i].se;
if(d+w<dist[v])
{
dist[v]=d+w;
pq.push(mp(dist[v],v));
}
}
}
if(dist[e]>=INF) dist[e]=-1;
cout<<dist[e]<<'\n';
}
