#ifdef __AVX2__
#pragma GCC target "avx2"
#endif
#pragma GCC optimize "O3"
#pragma GCC optimize "unroll-loops"
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define int long long
#define elif else if
#define all(l) begin(l),end(l)
#define rall(l) rbegin(l),rend(l)
#define append push_back
#define print(l) for(auto i:l) cout<<i<<' '; cout<<endl;
#define pprint(a,b) cout<<a<<' '<<b<<endl;
#define inp(l) for(auto &i:l) cin>>i;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define pai make_pair
#define endl "\n"
#define pii pair<int,int>
#define fi first
#define se second
#define vec vector
#define ins(a,b,c) a.insert(begin(a)+b,c);
// const int mod=998244353;
const int mod1=998244353;
const int mod=1e9+7;
const int N=2e5+5;
int iter=1,itera=1;
void solve(){
int n,m;
cin>>n>>m;
vec<vec<int>>pow(n);
int s=sqrt(n);
vec<vec<int>>dis(n,vec<int>(s+1,1e18));
vec<int>di(n+1,1e18);
int st,en;
for(int i=0;i<m;i++){
int b,p;
cin>>b>>p;
if(i==0) st=b;
if(i==1) en=b;
pow[b].append(p);
}
priority_queue<vec<int>,vec<vec<int>>,greater<>>q;
q.push({0,st,mod});
di[st]=0;
for(auto p:pow[st])
if(p<=s) dis[st][p]=0;
while(q.size()){
int u=q.top()[1],d=q.top()[0],ad=q.top()[2];
q.pop();
bool f=1;
if(ad<=s){
if(d==dis[u][ad]) pow[u].append(ad);
else f=0;
}
else if(d>di[u]) f=0;
if(!f) continue;
// cout<<u<<' '<<d<<' '<<ad<<endl;
for(auto po:pow[u]){
if(po<=s){
dis[u][po]=min(dis[u][po],d);
if(u>=po){
if(d+1<dis[u-po][po]){
di[u-po]=min(d+1,di[u-po]);
dis[u-po][po]=d+1;
q.push({d+1,u-po,po});
}
}
if(u+po<n){
if(d+1<dis[u+po][po]){
di[u+po]=min(d+1,di[u+po]);
dis[u+po][po]=d+1;
q.push({d+1,u+po,po});
}
}
}
else{
int tem=d;
for(int i=u-po;i>=0;i-=po){
tem++;
if(tem<di[i]){
q.push({tem,i,po});
di[i]=tem;
}
}
tem=d;
for(int i=u+po;i<n;i+=po){
tem++;
if(tem<di[i]){
q.push({tem,i,po});
di[i]=tem;
}
}
}
}
pow[u].clear();
}
if(di[en]>1e15) cout<<"-1\n";
else cout<<di[en]<<endl;
}
signed main(){
// freopen("","r",stdin);
// freopen("","w",stdout);
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
cout<<fixed<<setprecision(20);
// cin>>itera;
for(iter=1;iter<=itera;iter++) solve();
}
