#include <iostream>
#include <vector>
#include <queue>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pb push_back
#define endl '\n'
#define all(x) x.begin(),x.end()
#define yes cout<<"Yes"<<endl
#define no cout<<"No"<<endl
const int N = 30010,mod = 1e9+7;
int vstd[N],added[N];
pair<int,int>A[N];
vector<int> tile[N];
int main()
{
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
int n,m;
cin>>n>>m;
for(int i = 0;i<m;i++)
{
cin>>A[i].F>>A[i].S;
tile[A[i].F].pb(i);
}
priority_queue<pair<int,int>>pq;
for(auto u:tile[A[0].F])
{
pq.push({0,u});
added[u] = 1;
}
while(!pq.empty())
{
int nd = pq.top().S;
int jumps = -pq.top().F;
int x = A[nd].F;
int p = A[nd].S;
pq.pop();
added[nd] = 1;
if(vstd[nd])continue;
vstd[nd] = 1;
if(nd==1)
{
cout<<jumps<<endl;
return 0;
}
int k = 1;
for(int i = x+p;i<n;i+=p)
{
for(auto u:tile[i])
{
if(added[u])continue;
pq.push({-(k+jumps),u});
}
k++;
}
k = 1;
for(int i = x-p;i>=0;i-=p)
{
for(auto u:tile[i])
{
if(added[u])continue;
pq.push({-(k+jumps),u});
}
k++;
}
}
cout<<-1<<endl;
return 0;
}
/*
*/
