#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 = long long;
void solve(){
ll n, m;
cin >> n >> m;
vector<vector<pair<ll, ll>>> adj(n, vector<pair<ll, ll>>());
vector<pair<ll, ll>> list_m(m, {0, 0});
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++){
cin >> list_m[i].first;
cin >> list_m[i].second;
if(i == 0){
zero = list_m[i].first;
}else if(i == 1){
one = list_m[i].first;
}
list_n[list_m[i].first].push_back(i);
list_n2[list_m[i].first].push_back(list_m[i].second);
}
for(ll i = 0; i < n; i++){
for(ll j : list_n2[i]){
ll temp = i - j;
while(temp >= 0){
adj[i].push_back({temp, (i - temp)/j});
temp-=j;
}
temp = i + j;
while(temp < n){
adj[i].push_back({temp, (temp - i)/j});
temp+=j;
}
}
}
multiset<pair<ll, ll>> mult;
vector<ll> vis(n, 1E17);
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;
}
for(auto node : adj[top.second]){
if(vis[node.first] <= top.first + node.second){
continue;
}else{
vis[node.first] = top.first + node.second;
mult.insert({top.first + node.second, node.first});
}
}
}
if(vis[one] == 1E17){
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();
}
}
