#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize("-Ofast")
#include <bits/stdc++.h>
#include <algorithm>
#include <iostream>
#include <vector>
#include <limits>
#include <cmath>
#include <stack>
#include <queue>
#include <map>
#include <math.h>
using namespace std;
/*
* Score : 36/100
*
* Subtask 4 (21 points)
* Subtask 5 (43 points)
*/
using ll = int;
void solve(){
ll n, m;
cin >> n >> m;
//vector<vector<pair<ll, ll>>> adj(n, vector<pair<ll, ll>>());
vector<vector<ll>> list_n(n, vector<ll>());
vector<vector<ll>> list_n2(n, vector<ll>());
ll zero = 0;
ll one = 0;
for(ll i = 0; i < m; i++){
ll a, b;
cin >> a >> b;
if(i == 0){
zero = a;
}else if(i == 1){
one = a;
}
list_n[a].push_back(i);
list_n2[a].push_back(b);
}
for(ll i = 0; i < n; i++){
sort(list_n2[i].rbegin(), list_n2[i].rend());
}
multiset<pair<ll, ll>> mult;
vector<ll> vis(n, INT_MAX);
mult.insert({0, zero});
vis[zero] = 0;
while(!mult.empty()){
auto top = *(mult.begin());
mult.erase(mult.begin());
if(vis[top.second] < top.first){
continue;
}
ll i = top.second;
vector<bool> vis2(n, false);
for(ll j : list_n2[i]){
ll temp = i - j;
while(temp >= 0){
if(!vis2[temp]){
vis2[temp] = true;
if(vis[temp] <= top.first + (i - temp)/j){
continue;
}else{
vis[temp] = top.first + (i - temp)/j;
mult.insert({top.first + (i - temp)/j, temp});
}
if(temp == one){
break;
}
}
temp-=j;
}
temp = i + j;
while(temp < n){
if(!vis2[temp]){
vis2[temp] = true;
if(vis[temp] <= top.first + (temp - i)/j){
continue;
}else{
vis[temp] = top.first + (temp - i)/j;
mult.insert({top.first + (temp - i)/j, temp});
}
if(temp == one){
break;
}
}
temp+=j;
}
}
}
if(vis[one] == INT_MAX){
cout << -1 << endl;
}else{
cout << vis[one];
}
}
bool single = true;
signed main(){
ios_base::sync_with_stdio(false);
cout.tie(0);
cin.tie(0);
ll t = 1;
if(!single) cin >> t;
while(t--){
solve();
}
}
