#include <bits/stdc++.h>
using namespace std;
#define ll long long
int const N=3e4+5;
int const inf=1e9+7;
vector<int> sky[N];
int pw[N];
int pos[N];
set<pair<int,int>> vis;
int main(){
int n,m;
cin>>n>>m;
for (int i = 0; i < m; ++i){
cin>>pos[i]>>pw[i];
sky[pos[i]].push_back(pw[i]);
}
int tar=pos[1];
vector<int> in;
in.push_back(pos[0]);
vis.insert({pos[0],pw[0]});
int cur=0;
while(in.size()){
vector<pair<int,int>> upd;
for(int c:in){
// cout<<"sky := "<<c<<endl;
if(c==tar){
cout<<cur<<endl;
return 0;
}
for(int p:sky[c]){
// cout<<p<<' ';
if(c+p<n)
upd.push_back({c+p,p});
if(c-p>=0)
upd.push_back({c-p,p});
}
// cout<<endl;
sky[c].clear();
}
in.clear();
// cout<<"Upd"<<endl;
for(auto [s,p]:upd){
if(vis.find({s,p})!=vis.end())
continue;
// cout<<s<<' '<<p<<endl;
vis.insert({s,p});
sky[s].push_back(p);
in.push_back(s);
}
cur++;
// cout<<"next"<<endl;
}
cout<<-1<<endl;
return 0;
}
