#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<endl
#define dout(a) cout<<a<<' '<<#a<<endl
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<endl
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
const int mxn = 30005;
struct info{
int pos, doge, dist;
};
vi f(int x){
vi r;
FOR(i, 1, floor(sqrt(x) + 1)){
if(x % i == 0)r.pb(i);
}
int s = r.size();
f0r(i, s){
if(r[i] * r[i] != x){
r.pb(x / r[i]);
}
}
sort(rall(r));
return r;
}
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
vvi facts(mxn);
FOR(i, 1, mxn){
facts[i] = f(i);
}
//ifstream cin(".in");
//ofstream cout(".out");
in2(n,m);
vi b(m), p(m);
vector<set<int>>d(n);
f0r(i, m){
cin>>b[i]>>p[i]; d[b[i]].insert(p[i]);
}
vector<bitset<mxn>> vis(mxn);
vb viss(n);
queue<info>q;
for(auto u : d[b[0]]){
q.push({b[0], u, 0});
}
if(b[0] == b[1]){
out(0); return 0;
}
// vi dist(n,4e18); dist[b[0]] = 0;
while(!q.empty()){
int x = q.front().pos; int y = q.front().doge; int dis = q.front().dist; q.pop();
// out3(x,y,dis);
if(x + y == b[1] || x - y == b[1]){
out(dis + 1); return 0;
}
if(x + y < n){
if(!viss[x + y]){
viss[x + y] = 1;
for(auto u : d[x + y]){
if(!vis[x+y].test(u)){
q.push({x+y, u, dis + 1}); vis[x+y].set(u);
}
}
}
if(!vis[x+y].test(y)){
q.push({x+y, y, dis + 1}); vis[x+y].set(y);
}
}
if(x - y > -1){
if(!viss[x - y]){
viss[x - y] = 1;
for(auto u : d[x - y]){
if(!vis[x-y].test(u)){
q.push({x-y, u, dis + 1}); vis[x-y].set(u);
}
}
}
if(!vis[x-y].test(y)){
q.push({x-y, y, dis + 1}); vis[x-y].set(y);
}
}
}
out(-1);
}
