#include <bits/stdc++.h>
using namespace std;
#define name "jakarta"
#define MAXN 30004
#define pb push_back
#define pf push_front
#define ll long long
#define ii pair<int, int>
#define fs first
#define sc second
#define ill pair<int, ll>
#define lli pair<ll, int>
#define llll pair<ll, ll>
#define all(v) v.begin(),v.end()
#define uni(v) v.erase(unique(all(v)),v.end())
#define bit(n,i) (((n)>>(i))&1)
#define FOR(i,a,b) for (int i=(a),_b=(b); i<=_b; i++)
#define FORD(i,a,b) for (int i=(a),_b=(b); i>=_b; i--)
#define MASK(i) (1LL<<(i))
const int INF=1e9;
const int MOD=1e9+7;
void add(int &u, int v){
u+=v;
if (u>=MOD) u-=MOD;
}
void sub(int &u, int v){
u-=v;
if (u<0) u+=MOD;
}
void minimize(int &u, int v){
u=min(u,v);
}
void maximize(int &u, int v){
u=max(u,v);
}
long long Rand(long long l, long long r){
ll tmp=0;
FOR(i,1,4) tmp=((tmp<<15)^(((1<<15)-1)&rand()));
return l+tmp%(r-l+1);
}
#define iiii pair<ii,ii>
int n,m;
int b[MAXN],p[MAXN];
const int BLOCKS=175;
bool visited[MAXN][BLOCKS+1];
int dp[MAXN];
vector<int> vec[MAXN];
int main()
{
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
cin>>n>>m;
FOR(i,1,m) cin>>b[i]>>p[i];
FOR(i,1,m) ++b[i];
FOR(i,1,m) vec[b[i]].pb(p[i]);
memset(dp,0x3f,sizeof(dp));
dp[b[1]]=0;
queue<iiii> Q;
Q.push((iiii){{0,-1},{b[1],p[1]}});
if (p[1]<=BLOCKS) visited[b[1]][p[1]]=1;
while (Q.size()){
iiii tmp=Q.front();
Q.pop();
int u=tmp.sc.fs;
int w=tmp.sc.sc;
int d=tmp.fs.fs;
int prev=tmp.fs.sc;
while (1){
if (w<=BLOCKS){
int v;
v=u-w;
if (v!=prev&&v>=1&&!visited[v][w]){
visited[v][w]=1;
minimize(dp[v],d+1);
Q.push((iiii){{d+1,u},{v,w}});
}
v=u+w;
if (v!=prev&&v<=n&&!visited[v][w]){
visited[v][w]=1;
minimize(dp[v],d+1);
Q.push((iiii){{d+1,u},{v,w}});
}
}
else{
int v;
v=u-w;
if (v!=prev&&v>=1){
minimize(dp[v],d+1);
Q.push((iiii){{d+1,u},{v,w}});
}
v=u+w;
if (v!=prev&&v<=n){
minimize(dp[v],d+1);
Q.push((iiii){{d+1,u},{v,w}});
}
}
if (vec[u].size()==0) break;
w=vec[u].back();
vec[u].pop_back();
}
}
cout<<(dp[b[2]]==dp[0]?-1:dp[b[2]]);
return 0;
}
