#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=1e5;
const int mod=998244353;
const int mxlogN=40;
const int mxK=26;
const int inf=1e9;
const int K=600;
struct st
{
int x,p,d;
};
vector<int> who[mxN];
map<pair<int,int>, int> mp;
deque<st> q;
int n, m, a[mxN], b[mxN];
void check(int x, int p, int d, bool b)
{
if(x<0||x>=n) return;
if(!mp.count({x,p})||mp[{x,p}]>d)
{
mp[{x,p}]=d;
if(b) q.push_back({x,p,d});
else q.push_front({x,p,d});
}
}
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=1; i<m; i++) who[a[i]].push_back(i);
q.push_back({a[0],b[0],0});
mp[{a[0],b[0]}]=0;
while(q.size())
{
auto par=q.front(); q.pop_front();
int x=par.x,p=par.p,d=par.d;
if(d!=mp[{x,p}]) continue;
while(who[x].size())
{
int j=who[x].back(); who[x].pop_back();
if(j==1)
{
cout << d << "\n";
return 0;
}
int x2=a[j], p2=b[j], d2=d;
check(x2,p2,d2,0);
}
check(x-p,p,d+1,1);
check(x+p,p,d+1,1);
}
cout << "-1" << "\n";
return 0;
}
/*
6 2 2
8 1 2 1 5 4
*/
