//```// YF YUSUF
#define YF ios_base::sync_with_stdio(0);cout.setf(ios::fixed);
#define YUSUF cout.precision(0);cout.tie(0);cin.tie(0);
// #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 <ctime>
using namespace std;
#ifdef YF_CHECK
bool Output=1;
#else
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("O3")
bool Output=0;
#endif
using ll = long long;
using ld = long double;
using sll = set <ll>;
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 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;}
const ll mod=998244353;
const ll INF=1e18;
const ll inf=1e9+7;
const ll N =1e6+7;
void YF_MAIN(ll TEST){
int n,m;
cin>>n>>m;
vector<pair<int,int>> a(m);
for(auto &[b, p] : a){
cin>>b>>p;
}
if(a[0].F==a[1].F){
cout<<0;
return;
}
int sn = sqrt(n);
vector<vvl> edge(sn+3);
vector<vector<int>> pos(n+1);
for(int i=0;i<m;i++){
pos[a[i].F].pb(i);
}
ll s=0,f=1;
using pli = pair<ll,int>;
priority_queue <pli,vector<pli>,greater<pli> > q;
vector<ll> d(n+1,INF);
d[a[s].F]=0;
q.push({0,a[s].F});
while(q.size()){
auto [dist,b] = q.top();
q.pop();
if(b==a[f].F)break;
if(dist != d[b])continue;
for(auto i : pos[b]){
ll p = a[i].S;
if(p > sn){
for(int B = b%p; B<n; B+=p){
if(d[B] > d[b] + abs(b-B)/p){
d[B] = d[b] + abs(b-B)/p;
q.push({d[B], B});
}
}
}
else{
if(edge[p].empty()){
edge[p].resize(p+1, {});
for(int i=0;i<m;i++)
edge[p][a[i].F % p].pb(i);
}
for(auto j : edge[p][b % p]){
int bto = a[j].F;
if(d[bto] > d[b] + abs(bto-b)/p){
d[bto] = d[b] + abs(bto-b)/p;
q.push({d[bto], bto});
}
}
}
}
}
cout<<(d[a[f].F]==INF ? -1 : d[a[f].F]);
}
bool TECT=0;
bool OUTP=1;
ll lg(ll x){return (x ? 63 - __builtin_clzll(x) : -1);}
const ll SN=2e0 + 7; ll SM[SN];
const ll FN=2e0 + 7; ll FACT[FN], inv[FN], FMOD=inf;
ll PER(ll n,ll k){return FACT[n] *inv[n-k]%FMOD;}
ll CNK(ll n,ll k){return PER(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;
if(i<5e5+7)inv[i]=BP(FACT[i],FMOD-2,FMOD);
}
}
signed main(){
// freopen(("input.txt"),"r",stdin);freopen(("output.txt"),"w",stdout);
if(OUTP){YF YUSUF}
cout<<(0 && Output ? "\nYF_OUTPUT:\n\n" : "");
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;
}
//```
