#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;
vector<pll> graph[21000001];
ll dist[2100001];
vi dog_poz[sqr];
int cur_v;
int min_dist[30002];
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
int n,m;
cin >> n >> m;
cur_v = n;
vector<pll> elms(m);
rep(i,m) cin >> elms[i].ff >> elms[i].ss;
vi poz;
rep(i,m)
{
if(i == 1) continue;
if(elms[i].ss >= sqr)
{
int p = elms[i].ff%elms[i].ss;
poz = {};
while(p < n)
{
poz.pb(p);
p += elms[i].ss;
}
rep(j,siz(poz))
{
if(j != siz(poz)-1)
{
graph[cur_v].pb({cur_v+1,1});
graph[cur_v+1].pb({cur_v,1});
}
graph[cur_v].pb({poz[j],0});
if(poz[j] == elms[i].ff) graph[poz[j]].pb({cur_v,0});
cur_v++;
}
}
else dog_poz[elms[i].ss].pb(elms[i].ff);
}
rep2(p,1,sqr-1)
{
if(siz(dog_poz[p]) == 0) continue;
rep(i,n) min_dist[i] = 1e9;
forall(it,dog_poz[p]) min_dist[it] = 0;
rep(i,n) if(i-p >= 0) min_dist[i] = min(min_dist[i],min_dist[i-p]+1);
for(int i = n-1; i >= 0; i--) if(i+p < n) min_dist[i] = min(min_dist[i],min_dist[i+p]+1);
rep(i,p)
{
if(min_dist[i] == 1e9) continue;
int j = i;
while(j < n)
{
if(j+p < n)
{
graph[cur_v].pb({cur_v+1,1});
graph[cur_v+1].pb({cur_v,1});
}
graph[cur_v].pb({j,0});
if(min_dist[j] == 0) graph[j].pb({cur_v,0});
cur_v++;
j += p;
}
}
}
rep(i,cur_v) dist[i] = -1;
deque<pll> pq;
pq.push_front({0,elms[0].ff});
while(!pq.empty())
{
pll t = pq.front();
pq.pop_front();
if(dist[t.ss] != -1) continue;
dist[t.ss] = t.ff;
forall(it,graph[t.ss]) if(it.ss == 0) pq.push_front({t.ff,it.ff}); else pq.push_back({t.ff+1,it.ff});
}
cout << dist[elms[1].ff] << "\n";
}
