#include<bits/stdc++.h>
using namespace std;
#define endl "\n"
#define int long long
const int TAILLEMAXI=2002;
int dejavu[TAILLEMAXI][TAILLEMAXI]; ///endroit,P
vector<int> depart[TAILLEMAXI];
int nbvilles,nbjumpers;
vector<pair<int,int>> jumpers; /////endroit,P
deque<pair<int,int>> encours;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>nbvilles>>nbjumpers;
for (int i=0;i<nbjumpers;i++){
int a,b;
cin>>a>>b;
jumpers.push_back({a,b});
depart[a].push_back(b);
}
encours.push_back({jumpers[0].first,jumpers[0].second});
int posfinal=jumpers[1].first;
int rep=0;
while (encours.size()!=0){
int taille=encours.size();
int a=0;
while (a<taille){
int pos=encours.front().first,saut=encours.front().second;
encours.pop_front();
a++;
if (pos>=0 and pos<nbvilles and dejavu[pos][saut]==0){
dejavu[pos][saut]=1;
if (pos==posfinal){
cout<<rep<<endl;
return 0;
}
for (int i:depart[pos]){
if (i!=saut){
encours.push_front({pos,i});
a--;
}
}
encours.push_back({pos-saut,saut});
encours.push_back({pos+saut,saut});
}
}
rep++;
/*cout<<encours[0].first<<" "<<encours[0].second<<endl;
cout<<encours[1].first<<" "<<encours[1].second<<endl;
return 0;*/
}
cout<<-1<<endl;
return 0;
}
