#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vl ;
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define all(v) (v).begin(),(v).end()
const int MOD = 1000000007;
const int N = 1000005;
const double PI =4*atan(1);
const double eps = 1e-7;
ll n,m;
ll x,y;
vl tab[N];
ll p[N];
ll b[N];
bool vis[N];
set <pair<ll,ll> > sss;
ll ans=-1;
void bfs(ll u){
set<pair<ll,pair<ll,ll> > > ss;
for(auto t:tab[u]){
ss.insert(mp(0,mp(u,p[t])));
}
vis[u]=1;
pair<ll,pair<ll,ll> > ras;
while(!ss.empty()){
ras=*ss.begin();
ss.erase(ss.begin());
if(ras.s.f==b[1]){
ans=ras.f;
return ;
}
if(sss.count(ras.s))continue;
sss.insert(ras.s);
//cout << ras.s.f << " " << ras.s.s << endl;
if(ras.s.s+ras.s.f <n && sss.count(mp(ras.s.s+ras.s.f,ras.s.s))==0){
ss.insert(mp(ras.f+1,mp(ras.s.s+ras.s.f,ras.s.s)));
}
if(ras.s.f-ras.s.s >=0 && sss.count(mp(ras.s.f-ras.s.s,ras.s.s))==0){
ss.insert(mp(ras.f+1,mp(ras.s.f-ras.s.s,ras.s.s)));
}
if(vis[ras.s.f])continue;
for(auto t: tab[ras.s.f]){
ss.insert(mp(ras.f,mp(ras.s.f,p[t]) ));
}
vis[ras.s.f]=1;
}
}
int main(){
ios::sync_with_stdio(0);
//freopen("easy.txt","r",stdin);
cin >> n >>m ;
for(int i=0;i<m;i++){
cin >> x >> y;
p[i]=y;
b[i]=x;
tab[x].pb(i);
}
bfs(b[0]);
cout << ans;
return 0;
}
