#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
template <class T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define endl '\n';
#define fastio ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define py cout << "YES" << endl;
#define pn cout << "NO" << endl;
#define pb push_back
#define int long long
typedef long double lld;
#define double lld
typedef long long ll;
typedef unsigned long long ull;
const ll inf = 1e18;
const ll mod = 1e9+7;
const int N = 2e5;
vector<int>primes = {2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97};
// Check for queue, priorioty_queue, stack, ordered_set solutions
// stack => LIFO (whatever goes in last comes out last)
// queue => FIFO (whatever goes in first comes out first)
// priority_queue => Dynamic queries of minimum/maximum
// ordered_set => set in vector form
//[order_of_key(k) gives number of elements less than k, while find_by_order(i) gives i^th element]
// To comment multiple lines : ctrl + /
// To find and replace : ctrl+H
void judge(){
srand(time(NULL));
#ifndef ONLINE_JUDGE
freopen("1.txt","r",stdin);
freopen("2.txt","w",stdout);
freopen("Error.txt", "w", stderr);
#define debug(x...) cerr << #x <<" = "; _print(x); cerr << endl;
#else
#define debug(x...)
#endif
}
void usaco(string s) {
freopen((s + ".in").c_str(),"r",stdin);
freopen((s + ".out").c_str(),"w",stdout);
}
int n,m;
vector<vector<pair<int,int>>>adj;
vector<vector<int>>dist;
vector<int>b,p;
void input(){
cin >> n >> m;
adj.resize(n);
dist.resize(n,vector<int>(n,inf));
b.resize(m);
p.resize(m);
for(int i = 0;i<m;i++){
cin >> b[i] >> p[i];
}
}
void shortest_distance(int src, int trgt){
vector<int>dist(n,inf);
priority_queue<pair<int,int>>pq;
pq.push({0,src});
dist[src] = 0;
while(!pq.empty()){
auto x = pq.top(); pq.pop();
int d = -1*x.first, node = x.second;
if(dist[node]!=d){
continue;
}
for(auto [child,wt] : adj[node]){
if(dist[child]>dist[node]+wt){
dist[child] = dist[node]+wt;
pq.push({-1*dist[child],child});
}
}
}
if(dist[trgt]==inf){
cout << -1;
}
else{
cout << dist[trgt];
}
}
signed main(){
fastio; //judge();
input();
for(int i = 0;i<m;i++){
for(int j = b[i]%p[i];j<n;j+=p[i]){
if(j==b[i]){
continue;
}
int k = abs(j-b[i])/p[i];
dist[b[i]][j] = min(dist[b[i]][j],k);
}
}
for(int i = 0;i<n;i++){
for(int j = 0;j<n;j++){
if(dist[i][j]!=inf){
adj[i].pb({j,dist[i][j]});
}
}
}
shortest_distance(b[0],b[1]);
}
