#include<bits/stdc++.h>
#include<chrono>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
typedef string st;
typedef bool bl;
typedef vector<ll> vii;
typedef pair<ll,ll> pii;
typedef vector<pii> vpi;
#define pu push
#define ordered_set tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>
#define fast ios_base::sync_with_stdio(0);cin.tie();
#define test ll qqqqq;cin>>qqqqq;while(qqqqq--)
#define F first
#define S second
#define forn(i,n) for(ll i=0;i<n;i++)
#define forx(i,j,n) for(ll i=j;i<n;i++)
#define pb push_back
#define pob pop_back
#define all(v) v.begin(),v.end()
#define lb lower_bound
#define ub upper_bound
#define pof pop_front
#define pow powww
#define prtll(x) printf("%lld",x)
#define prtld(x) printf("%Lf",x)
#define prtst(x) printf("%s",x)
#define prtch(x) printf("%c",x)
#define measure chrono::high_resolution_clock::now()
const ll dx[]{1,0,-1,0};
const ll dy[]{0,-1,0,1};
const ll inf=2e18;
const ll mod=1e9+7;
const ll LM=2e7+1;
const ll M=2e6+1;
const ll MM=3002;
const ll MMM=501;
const ld pi=acos(-1);
//const ll mod=998244353;
ll pow(ll r,ll to,ll m=mod){
ll res=1;
r%=mod;
while(to){
if((to&1)){
res*=r,res%=mod;
}
r*=r,r%=mod;
to=(to>>1);
}
return res;
}
ll n,m,a[M],p[M],dis[MM][MM],ind0,ind1,dist[M];
vii v[M];
vpi vv[M];
void dij(ll ind){
priority_queue<pii,vpi,greater<pii>>q;
q.push({0,ind});
dist[ind]=0;
while(!q.empty()){
pii x=q.top();
q.pop();
for(auto it:vv[x.S]){
if(dist[it.F]>x.F+it.S)
q.push({x.F+it.S,it.F}),dist[it.F]=x.F+it.S;
}
}
}
int main(){
fast
memset(dis,mod,sizeof dis);
memset(dist,mod,sizeof dist);
cin>>n>>m;
forn(i,m){
cin>>a[i]>>p[i];
v[a[i]].pb(p[i]);
if(i==0)
ind0=a[i];
if(i==1)
ind1=a[i];
}
forn(i,n){
forn(j,n){
for(auto it:v[i]){
if(abs(i-j)%it==0)
dis[i][j]=min(dis[i][j],abs(i-j)/it);
}
}
}
forn(i,n){
forn(j,n){
if(dis[i][j]<mod)
vv[i].pb({j,dis[i][j]});
}
}
dij(ind0);
if(dist[ind1]>=mod)
dist[ind1]=-1;
cout<<dist[ind1];
return 0;
}
/*
*/
