#include <bits/stdc++.h>
using namespace std;
#define _Thich_Ngu_Trua_ int main()
#define Siesta ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ll long long
#define pii pair<ll, ll>
#define F first
#define S second
const ll N = 1000005;
const ll inf = 1e18;
const ll mod = 1e9+7;
ll n, m, pos[N], stren[N], mx = -inf, mn = inf;
vector<ll> dist(N, inf);
map<ll, vector<ll>> mp;
vector<pii> adj[N];
void djk()
{
priority_queue<pii, vector<pii>, greater<pii>> pq;
dist[0] = 0;
pq.push({0, 0});
while(!pq.empty())
{
auto [val, u] = pq.top();
pq.pop();
if(val != dist[u]) continue;
for(auto [v, w]:adj[u])
{
if(dist[v] > dist[u] + w)
{
dist[v] = dist[u] + w;
pq.push({dist[v], v});
}
}
}
cout << (dist[1]==inf?-1:dist[1]);
}
_Thich_Ngu_Trua_
{
if (fopen("file.inp", "r"))
{
freopen("file.inp", "r", stdin);
freopen("file.out", "w", stdout);
}
Siesta;
cin >> n >> m;
for(ll i=0; i<m; i++)
{
cin >> pos[i] >> stren[i];
mp[pos[i]].push_back(i);
if(pos[i] > mx) mx = pos[i];
if(pos[i] < mn) mn = pos[i];
}
for(ll i=0; i<m; i++)
{
ll st = pos[i];
for(ll j=1; st+j*stren[i]<=mx; j++)
{
if(!mp.count(st+j*stren[i])) continue;
for(ll v:mp[st+j*stren[i]])
{
adj[i].push_back({v, j});
}
}
for(ll j=1; st-j*stren[i]>=mn; j++)
{
if(!mp.count(st-j*stren[i])) continue;
for(ll v:mp[st-j*stren[i]])
{
adj[i].push_back({v, j});
}
}
}
djk();
}
