#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
void solve(){
int n, m;
cin >> n >> m;
pair < int , int > doge[m];
vector < int > has[n];
for(int i = 0;i<m;i++){
cin >> doge[i].first >> doge[i].second;
has[doge[i].first].push_back(doge[i].second);
}
int edge[n][n];
for(int i = 0;i<n;i++){
for(int j = 0;j<n;j++){
edge[i][j] = inf;
}
}
for(int i = 0;i<n;i++){
sort(has[i].begin() , has[i].end());
has[i].resize(unique(has[i].begin() , has[i].end()) - has[i].begin());
for(auto itr : has[i]){
for(int j = i;j<n;j+=itr){
edge[i][j] = min(edge[i][j] , (j-i) / itr);
}
for(int j = i;j>=0;j-=itr){
edge[i][j] = min(edge[i][j] , (i-j) / itr);
}
}
}
priority_queue < pair < int , int > , vector < pair < int , int > > , greater < pair < int , int > > > pq;
pq.push({0,0});
int vis[n];
memset(vis , 0 , sizeof(vis));
while(pq.size()){
pair < int , int > tmp = pq.top();
pq.pop();
if(vis[tmp.second])continue;
vis[tmp.second] = 1;
if(tmp.second == 1){
cout << tmp.first << endl;
return;
}
for(int i = 0;i<n;i++){
if(edge[tmp.second][i] != inf and vis[i] == 0){
pq.push({tmp.first + edge[tmp.second][i] , i});
}
}
}
cout << -1 << endl;
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);
int testcase = 1;//cin >> testcase;
while(testcase--)solve();
}
