#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define int long long
typedef pair<int,int> pii;
typedef vector<pii> vii;
typedef vector<int> vi;
typedef vector<bool> vb;
#define FOR(i,a) for(int i=0;i<(a);i++)
#define FORE(i,a,b) for(int i=(a);i<(b);i++)
#define all(x) x.begin(),x.end()
#define fi first
#define se second
#define pb push_back
#define sp <<" "<<
#define cont(x) for(auto el:x) cout<<el<<' ';cout<<endl;
#define contp(x) for(auto el:x) cout<<el.fi<<'-'<<el.se<<' ';cout<<endl;
#define DEBUG(x) cout<<#x sp x<<endl;
#define carp(x,y) ((x%MOD)*(y%MOD))%MOD
#define topla(x,y) ((x%MOD)+(y%MOD))%MOD
#define mid (l+r)/2
const int MAXN=3e4+5;
const int MOD=1e9+7;
const int INF=1e18;
int b[MAXN], p[MAXN];
int d[MAXN];
vi tut[MAXN];
int n,m;
void dij(){
set<pii> s; //{cost,hangi buildingdeyim}
s.insert({0,b[0]});
FOR(i,n) d[i]=INF;
d[b[0]]=0;
while(!s.empty()){
auto el=*s.begin();
s.erase(el);
int bina=el.se;
for(auto pp:tut[bina]){ //bu binadan kaç zıplamayla ayrılabilirim
int cost=0;
for(int i=bina+pp;i<n;i+=pp){
cost++;
if(d[i]>d[bina]+cost){
if(d[i]!=INF) s.erase({d[i],i});
d[i]=d[bina]+cost;
s.insert({d[i],i});
}
}
cost=0;
for(int i=bina-pp;i>=0;i-=pp){
cost++;
if(d[i]>d[bina]+cost){
if(d[i]!=INF) s.erase({d[i],i});
d[i]=d[bina]+cost;
s.insert({d[i],i});
}
}
}
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n>>m;
FOR(i,m){
cin>>b[i]>>p[i];
tut[b[i]].pb(p[i]);
}
dij();
cout<<(d[b[1]]==INF?-1:d[b[1]])<<endl;
}
