#include <bits/stdc++.h>
#include <iostream>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<ll,null_type,less_equal<ll>,rb_tree_tag,tree_order_statistics_node_update> order_set;
mt19937 mt_rand(chrono::high_resolution_clock::now().time_since_epoch().count());
//uniform_int_distribution<int> gen; ///(min, max)
//int random() {return gen(mt_rand);}
const int mxN=30001;
const int mod=998244353;
const int mxlogN=40;
const int mxK=26;
const int inf=1e9;
const int K=600;
int vis[6000000];
bool done[6000000];
int a[mxN], b[mxN],n,m;
vector<int> have, who2[mxN];
vector<pair<int,int>> who[mxN];
int pos(int x)
{
return lower_bound(have.begin(),have.end(),x)-have.begin();
}
struct st
{
int x,p,i;
};
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for(int i=0; i<m; i++) cin >> a[i] >> b[i];
for(int i=0; i<m; i++)
{
who[a[i]].push_back({a[i],b[i]});
who2[b[i]].push_back(a[i]%b[i]);
}
for(int i=1; i<mxN; i++)
{
sort(who2[i].begin(),who2[i].end());
who2[i].erase(unique(who2[i].begin(),who2[i].end()),who2[i].end());
for(int k=0; k*i<n; k++) for(int j:who2[i]) if(j+k*i<n) have.push_back(i*n+j+k*i);
}
for(int i=0; i<n; i++) for(auto& par:who[i]) par.first=pos(par.second*n+par.first);
deque<st> q; q.push_back({a[0],b[0],pos(b[0]*n+a[0])});
int sz=have.size();
for(int i=0; i<sz; i++) vis[i]=inf;
vis[pos(b[0]*n+a[0])]=0;
while(q.size())
{
auto cur=q.front(); q.pop_front();
int x=cur.x,p=cur.p,i=cur.i;
if(x==a[1])
{
cout << vis[i] << "\n";
return 0;
}
if(done[i]) continue;
done[i]=1;
while(who[x].size())
{
auto cur2=who[x].back(); who[x].pop_back();
int nw=cur2.first;
if(vis[i]<vis[nw])
{
vis[nw]=vis[i];
q.push_front({x,cur2.second,nw});
}
}
if(x-p>=0)
{
int nw=i-who2[p].size();
if(vis[i]+1<vis[nw])
{
vis[nw]=vis[i]+1;
q.push_back({x-p,p,nw});
}
}
if(x+p<n)
{
int nw=i+who2[p].size();
if(vis[i]+1<vis[nw])
{
vis[nw]=vis[i]+1;
q.push_back({x+p,p,nw});
}
}
}
cout << "-1" << "\n";
return 0;
}
/*
5 3
0 2
1 1
4 1
*/
