#include <bits/stdc++.h>
#define ll long long
#define F first
#define S second
#define pii pair<int, int>
using namespace std;
vector<vector<pii>>v;
int pos[2];
vector<int>dist;
bool operator<(pair<int,int>a,pair<int,int>b){
return a.S > b.S;
}
int n,m;
int conv(int q,int p){
return p * n + q;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >>n>>m;
int buck = 250;
dist.resize(n * buck + 2 * n);
v.resize(n * buck + 2 * n);
for(int i = 0;i < m;i++){
int q,p;
cin >>q>>p;
if(i < 2) pos[i] = q;
if(p <= buck){
v[conv(q,0)].push_back({conv(q,p),0});
}else{
int cnt = 1;
for(int j = q - p;j >= 0;j -= p){
v[conv(q,0)].push_back({conv(j,0),cnt});
cnt++;
}
cnt = 1;
for(int j = q + p;j < n;j += p){
v[conv(q,0)].push_back({conv(j,0),cnt});
cnt++;
}
}
}
for(int i = 0;i < n;i++){
for(int p = 0;p <= buck;p++){
dist[conv(i,p)] = 1e9;
}
}
dist[pos[0]] = 0;
priority_queue<pair<int,int>>q;
q.push({pos[0],0});
while(q.size()){
auto u = q.top();
// cout<<u.F<<' '<<u.S<<'\n';
q.pop();
if(dist[u.F] < u.S) continue;
for(auto x : v[u.F]){
if(dist[x.F] > u.S + x.S){
dist[x.F] = u.S + x.S;
q.push({x.F,dist[x.F]});
}
}
pii x;
if(u.F >= n){
x = {u.F + u.F / n,1};
if(x.F / n == u.F / n && dist[x.F] > u.S + x.S){
dist[x.F] = u.S + x.S;
q.push({x.F,dist[x.F]});
}
x = {u.F - u.F / n,1};
if(x.F / n == u.F / n && dist[x.F] > u.S + x.S){
dist[x.F] = u.S + x.S;
q.push({x.F,dist[x.F]});
}
x = {u.F % n,0};
if(dist[x.F] > u.S + x.S){
dist[x.F] = u.S + x.S;
q.push({x.F,dist[x.F]});
}
}
}
cout<<(dist[pos[1]] == 1e9 ? -1 : dist[pos[1]]);
}
