#include <bits/stdc++.h>
using namespace std;
const int M = 3e4 + 7;
bitset < 30007 > vis[M];
void solve(){
int n, m;
cin >> n >> m;
pair < int , int > doge[m];
vector < int > root[M];
for(int i = 0;i<m;i++){
cin >> doge[i].first >> doge[i].second;
root[doge[i].first].push_back(doge[i].second);
}
queue < pair < int , pair < int , int > > > pq;//dist , konum , zıplayış
pq.push({0,{doge[0].first,0}});
while(pq.size()){
pair < int , pair < int , int > > tmp = pq.front();
pq.pop();
if(vis[tmp.second.first][tmp.second.second] == 1){
continue;
}
if(tmp.second.first == doge[1].first){
cout << tmp.first << '\n';
return;
}
vis[tmp.second.first][tmp.second.second] = 1;
if(tmp.second.first + tmp.second.second < n)pq.push({tmp.first + 1 , {tmp.second.first + tmp.second.second , tmp.second.second}});
if(tmp.second.first - tmp.second.second >= 0)pq.push({tmp.first + 1 , {tmp.second.first - tmp.second.second , tmp.second.second}});
for(auto itr : root[tmp.second.first]){
if(itr == tmp.second.second)continue;
if(tmp.second.first + itr < n)pq.push({tmp.first + 1 , {tmp.second.first + itr , itr}});
if(tmp.second.first - itr >= 0)pq.push({tmp.first + 1 , {tmp.second.first - itr , itr}});
}
}
cout << -1 << '\n';
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int testcase = 1;//cin >> testcase;
while(testcase--)solve();
}
