#include <bits/stdc++.h>
#define ll long long
#define F first
#define S second
#define pii pair<int, int>
const int mxN = 2e6 + 5;
using namespace std;
vector<pii>v[mxN];
int pos[mxN];
int dist[mxN];
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 = 50;
for(int p = 1;p <= buck;p++){
for(int i = 0;i < n;i++){
if(i + p < n){
v[conv(i,p)].push_back({conv(i + p,p),1});
v[conv(i + p,p)].push_back({conv(i,p),1});
}
v[conv(i,p)].push_back({conv(i,0),0});
}
}
for(int i = 0;i < m;i++){
int q,p;
cin >>q>>p;
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.q<<' '<<u.F.p<<' '<<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]});
}
}
}
cout<<(dist[pos[1]] == 1e9 ? -1 : dist[pos[1]]);
}
