//by szh
#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pb push_back
#define debug(x) cerr<<#x<<"="<<x<<endl
#define pq priority_queue
#define inf 0x3f
#define rep(i,a,b) for (int i=a;i<(b);i++)
#define MP make_pair
#define SZ(x) (int(x.size()))
#define ll long long
#define mod 1000000007
#define ALL(x) x.begin(),x.end()
void inc(int &a,int b) {a=(a+b)%mod;}
void dec(int &a,int b) {a=(a-b+mod)%mod;}
int lowbit(int x) {return x&(-x);}
ll p0w(ll base,ll p) {ll ret=1;while(p>0){if (p%2ll==1ll) ret=ret*base%mod;base=base*base%mod;p/=2ll;}return ret;}
const int maxn = 3e4+10;
int n,m;
vector <int> power[maxn];
deque <pair<int,pii>> q;
bitset <maxn> vis[maxn];
int main() {
// freopen("input.txt","r",stdin);
std::ios::sync_with_stdio(false);cin.tie();
cin>>n>>m;
int des;
rep(i,0,m) {
int x,y;
cin>>x>>y;
power[x].pb(y);
if (i==0) q.push_back({0,{x,y}});
if (i==1) des=x;
}
while (!q.empty()) {
int d = q.front().fi;
int x = q.front().se.fi, y = q.front().se.se;
q.pop_front();
if (vis[x][y]==1) continue;
// cout<<x<<" "<<y<<endl;
vis[x][y] = 1;
if (x==des) {
cout<<d;
return 0;
}
if (x+y<n and vis[x+y][y]==0) q.push_back({d+1,{x+y,y}});
if (x-y>=0 and vis[x-y][y]==0) q.push_back({d+1,{x-y,y}});
for (auto it:power[x]) {
if (vis[x][it]==0) q.push_front({d,{x,it}});
}
}
cout<<-1;
return 0;
}
