#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;
set<int> cur;
FOR(i,n) cur.insert(i);
while(!s.empty()){
auto el=*s.begin();
s.erase(el);
int bina=el.se;
if(bina==b[1]) break;
cur.erase(bina);
/*cout<<"here" sp bina<<endl;
cont(tut[bina]);
cont(cur);*/
for(auto pp:tut[bina]){ //bu binadan kaç zıplamayla ayrılabilirim
int git=bina+pp;
int cost=1;
while(git<n){
auto it=cur.lower_bound(git);
if(it==cur.end()) break;
if(*it==git){
tut[git].pb(pp);
if(d[git]>d[bina]+cost){
if(s.count({d[git],git})) s.erase({d[git],git});
d[git]=d[bina]+cost;
s.insert({d[git],git});
}
break;
}
else{
cost=ceil((double)(*it-bina)/pp);
git=bina+pp*cost;
}
}
git=bina-pp;
cost=1;
while(git>=0){
//cout<<"git" sp git sp cost<<endl;
auto it=cur.lower_bound(git);
if(it!=cur.end() and *it==git){
tut[git].pb(pp);
if(d[git]>d[bina]+cost){
if(s.count({d[git],git})) s.erase({d[git],git});
d[git]=d[bina]+cost;
s.insert({d[git],git});
}
break;
}
else if(it==cur.begin()) break;
else{
it--;
cost=ceil((double)(bina-*it)/pp);
git=bina-pp*cost;
}
}
}
tut[bina].clear();
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.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;
}
