//```// YF YUSUF
#include <bits/stdc++.h>
// #include <iostream>
// #include <vector>
// #include <set>
// #include <map>
// #include <algorithm>
// #include <cmath>
// #include <numeric>
// #include <queue>
// #include <stack>
// #include <cassert>
// #include <climits>
// #include <string>
// #include <cstdlib>
// #include <random>
// #include <iomanip>
// #include <ctime>
using namespace std;
#ifdef YF_CHECK
bool LOCAL = 1;
#else
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("O3")
bool LOCAL = 0;
#endif
using ll = int;
using ld = long double;
using vll = vector <ll>;
using mll = map <ll,ll>;
using pll = pair <ll,ll>;
using vvl = vector <vll>;
using vpll = vector <pll>;
template<class T>T MIN(T&a,T b){a=min(a,b);return a;}
template<class T>T MAX(T&a,T b){a=max(a,b);return a;}
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define sgr v+v+1,(tl+tr)/2+1,tr
#define sgl v+v,tl,(tl+tr)/2
#define pb push_back
#define ins insert
#define S second
#define F first
mt19937_64 MT(time(0));
ll BP(ll a,ll b,ll mod=1e9+7){
if(b==0)return 1;
ll q=BP(a,b/2,mod);
return ((q*q)%mod*(b%2?a:1ll))%mod;
}
ll f(ll x){return x*(x+1)/2;}
ll dup(ll a,ll b){return (a+b-1)/b;}
ll lcm(ll a,ll b){return a/__gcd(a,b)*b;}
ll invf(ll x){return (-1+sqrt(1+8*x))/2;}
ll lg(ll x){return (x ? 63 - __builtin_clzll(x) : -1);}
const ll mod=998244353;
const ll INF=1e18;
const ll inf=1e9+7;
const ll N =5220175 +7;
ll n, m, ns, d[N];
bool exist[N];
vector<pll> g[N];
vpll a;
void pout(pll a){cout<<a.F<<" "<<a.S;}
pll DH(ll h) {return {h/ns, h%ns};}
ll H(ll a,ll b){
if(a * ns + b >= N){
return 0;
}
return a * ns + b;
}
bool make_edge(int v, int to, int w = 1){
// if(g[v].count({w, to}))return 0;
g[v].pb({w, to});
return 1;
}
vll P[30007];
void YF_MAIN(ll TEST){
cin>>n>>m;
a.resize(m);
ns = sqrt(n) + 1;
for(auto &[b, p] : a){
cin>>b>>p;
++b;
if(0>=p || p>=n)continue;
if(p <= sqrt(n)){
exist[H(b, p)] = 1;
make_edge(H(b, p), H(b, 0), 0);
make_edge(H(b, 0), H(b, p), 0);
} else{
P[b].pb(p);
}
}
for(int i=1;i<=n;i++){
sort(all(P[i]));
P[i].erase(unique(all(P[i])), P[i].end());
}
ll s=H(a[0].F, 0), f=H(a[1].F, 0);
priority_queue <pll,vector<pll>,greater<pll> > q;
for(int i=0;i<N;i++)d[i] = inf;
d[s]=0;
q.push({0, s});
bool out = 0;
auto powered = [&](int v, int to, int pos){
if(!exist[to] || d[to] > d[v] + 1){
exist[to] = 1;
make_edge(to, H(pos, 0), 0);
d[to]=d[v]+1;
q.push({d[to],to});
}
};
while(q.size()){
auto [dist,v] = q.top();
q.pop();
if(dist != d[v])continue;
// cout<<dist<<" "<<v<<"\n";
for(auto [dto,to] : g[v]){
if(d[to] > d[v] + dto){
d[to]=d[v]+dto;
q.push({d[to],to});
}
}
auto [b, p] = DH(v);
if(!p){
for(auto p : P[b]){
for(int to = b % p; to <= n; to+=p){
if(1 <= to && to <= n){
ll TO = H(to, 0);
if(d[TO] > d[v] + abs(to - b) / p){
d[TO] = d[v] + abs(to - b) / p;
q.push({d[TO], TO});
}
}
}
}
}
else if(exist[v] && p){
if(1 <= b-p) powered(v, H(b-p, p), b-p);
if(b+p <= n) powered(v, H(b+p, p), b+p);
}
}
cout<<(d[f] == inf ? -1 : d[f]);
}
const bool TECT=0;
const bool FLSH=0;
// const ll SN=1e0 + 7; ll SM[SN];
// const ll FN=1e0 + 7; ll FACT[FN], inv[FN], FMOD=inf;
// ll PNK(ll n,ll k){return FACT[n] *inv[n-k]%FMOD;}
// ll CNK(ll n,ll k){return PNK(n,k)*inv[k ]%FMOD;}
void BEFORE(){
// for(ll i=2;i<SN;i++){
// if(SM[i])continue;
// for(ll j=i;j<SN;j+=i)
// MAX(SM[j],i);
// }
// FACT[0]=inv[0]=1;
// for(int i=1;i<FN;i++){
// FACT[i]=FACT[i-1]*i%FMOD;
// inv[i]=BP(FACT[i],FMOD-2,FMOD);
// }
}
signed main(){
// freopen(("input.txt"),"r",stdin);freopen(("output.txt"),"w",stdout);
if(FLSH){
ios_base::sync_with_stdio(0);
cout.setf(ios::fixed);
cout.precision(0);
cout.tie(0);
cin.tie(0);
}
int TEST=1;
if(TECT)
cin>>TEST;
BEFORE();
for(int T=1;T<=TEST;T++){
// cout<<"Case "<<T<<": ";
YF_MAIN(T);
cout<<(T==TEST ? "" : "\n");
}
return 0;
}
// YF YUSUF ```
