#include <bits/stdc++.h>
#define startt ios_base::sync_with_stdio(false);cin.tie(0);
typedef long long ll;
using namespace std;
#define vint vector<int>
#define all(v) v.begin(), v.end()
#define MOD 1000000007
#define MOD2 998244353
#define MX 1000000000
#define MXL 1000000000000000000
#define PI (ld)2*acos(0.0)
#define pb push_back
#define sc second
#define fr first
//#define int long long
#define endl '\n'
#define ld long double
#define NO cout << "NO" << endl
#define YES cout << "YES" << endl
//mesanu
int mp[30000][175];
vector<pair<int, int>> adj[30000*175];
int now = 1;
void cnn(int first, int second)
{
mp[first][second] = now;
now++;
}
int dijkstra(int s, int e)
{
int n = now;
vector<int> d(n, MX);
d[s] = 0;
priority_queue<pair<int, int>> q;
q.push({0, s});
while (!q.empty()) {
int v = q.top().second;
int dist = -q.top().first;
q.pop();
if(dist != d[v])
{
continue;
}
for (auto edge : adj[v]) {
int to = edge.first;
int len = edge.second;
if (d[v] + len < d[to]) {
d[to] = d[v] + len;
q.push({-d[to], to});
}
}
}
return d[e];
}
void solve()
{
int n, m;
cin >> n >> m;
int start = 0;
int end = 0;
int th = sqrt(n);
for(int sky = 0; sky < n; sky++)
{
cnn(sky, 0);
}
map<pair<int, int>, bool> used;
for(int i = 0; i < m; i++)
{
int sky, power;
cin >> sky >> power;
if(i == 0)
{
start = sky;
}
if(i == 1)
{
end = sky;
}
if(used[{sky,power}])
{
continue;
}
used[{sky, power}] = true;
if(power <= th)
{
if(mp[sky][power] != 0)
{
adj[mp[sky][0]].pb({mp[sky][power], 0});
continue;
}
for(int j = sky%power; j < n; j+=power)
{
if(mp[j][power] == 0)
{
cnn(j, power);
}
adj[mp[j][power]].pb({mp[j][0], 0});
if(j+power < n)
{
cnn(j+power, power);
adj[mp[j][power]].pb({mp[j+power][power], 1});
adj[mp[j+power][power]].pb({mp[j][power], 1});
}
}
adj[mp[sky][0]].pb({mp[sky][power], 0});
}
else
{
for(int j = sky%power; j < n; j+=power)
{
adj[mp[sky][0]].pb({mp[j][0], abs(sky-j)/power});
}
}
}
used.clear();
int dist = dijkstra(mp[start][0], mp[end][0]);
if(dist == MX)
{
cout << -1 << endl;
}
else
{
cout << dist << endl;
}
}
int32_t main(){
startt
int t = 1;
//cin >> t;
while (t--) {
solve();
}
}
/*
5 3
0 2
1 1
4 1
*/
