#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
const int sqr = 174;
vi dists[22000001];
bool is_p[30001][sqr];
bool was[30003];
set<int> ps[30001];
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
int n,m;
cin >> n >> m;
vector<pll> elms(m);
rep(i,m) cin >> elms[i].ff >> elms[i].ss;
rep(i,m) if(i != 1 && elms[i].ss < sqr) is_p[elms[i].ff][elms[i].ss] = 1;
rep(i,m) if(i != 1) ps[elms[i].ff].insert(elms[i].ss);
dists[0].pb(elms[0].ff);
rep2(d,0,12000000) forall(it,dists[d])
{
if(was[it]) continue;
was[it] = 1;
if(it == elms[1].ff)
{
cout << d << "\n";
return 0;
}
forall(p,ps[it])
{
if(p >= sqr)
{
rep2(x,1,sqr+1) if(it+p*x < n)
{
if(!was[it+p*x]) dists[d+x].pb(it+p*x);
} else break;
for(int x = -1; x >= -sqr-1; x--) if(it+p*x >= 0)
{
if(!was[it+p*x]) dists[d-x].pb(it+p*x);
} else break;
}
else
{
int x = it+p;
while(x < n)
{
if(!was[x]) dists[d+abs(it-x)/p].pb(x);
if(is_p[x][p]) break;
x += p;
}
x = it-p;
while(x >= 0)
{
if(!was[x]) dists[d+abs(it-x)/p].pb(x);
if(is_p[x][p]) break;
x -= p;
}
}
}
}
cout << -1 << "\n";
}
