#include <iostream>
#include <bits/stdc++.h>
using namespace std;
typedef int64_t llo;
#define mp make_pair
#define pb push_back
#define a first
#define b second
int n,m;
int aa,bb;
const int ss=130;
vector<pair<pair<int,int>,int>> adj[30001][ss];//((index,power),dist)
int dist[30001][ss];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>n>>m;
int st;
int endd;
memset(dist,-1,sizeof(dist));
for(int j=0;j<m;j++){
cin>>aa>>bb;
if(j==0){
st=aa;
}
if(j==1){
endd=aa;
}
if(bb>=ss){
for(int i=aa+bb;i<n;i+=bb){
adj[aa][0].pb({{i,0},(i-aa)/bb});
}
for(int i=aa-bb;i>=0;i-=bb){
adj[aa][0].pb({{i,0},(aa-i)/bb});
}
}
else{
adj[aa][0].pb({{aa,bb},0});
// adj[aa][bb].pb({{aa,0},0});
}
}
for(int i=0;i<n;i++){
for(int j=1;j<ss;j++){
if(i+j<n){
adj[i][j].pb({{i+j,j},1});
}
if(i-j>=0 and i-j<n){
adj[i][j].pb({{i-j,j},1});
}
adj[i][j].pb({{i,0},0});
}
}
priority_queue<pair<int,pair<int,int>>> ac;
ac.push({0,{st,0}});
dist[st][0]=0;
while(ac.size()){
pair<int,pair<int,int>> no=ac.top();
ac.pop();
no.a=-no.a;
if(dist[no.b.a][no.b.b]<no.a){
continue;
}
// cout<<no.b.a<<","<<no.b.b<<","<<no.a<<endl;
for(auto j:adj[no.b.a][no.b.b]){
if(dist[j.a.a][j.a.b]==-1 or dist[j.a.a][j.a.b]>dist[no.b.a][no.b.b]+j.b){
dist[j.a.a][j.a.b]=dist[no.b.a][no.b.b]+j.b;
ac.push({-dist[j.a.a][j.a.b],{j.a}});
}
}
}
cout<<dist[endd][0]<<endl;
return 0;
}
