// solstice
// created at 31 january 2025
// sussy baka
/*
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠠⠖⠀⠀⢀⣨⣤⡤⠶⠶⠶⠶⢦⣤⣤⣀⠐⠢⢤⣀⠀⠀⠀⠀⠀⠀⠀⡆⣿⠀⠀⣀⠈⢷⡘⣷⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⡴⠚⠉⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠉⠓⠦⣌⡙⠶⣄⠀⠀⠀⢠⠀⣿⠀⢠⡇⠀⠈⡇⢸⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠶⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⠷⣌⠻⣦⣠⠏⣸⠃⢀⣎⡇⠀⢠⡇⣼⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠞⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡀⢀⡴⠶⣄⠀⠀⠀⠀⠈⠳⣌⡙⢰⠇⢀⣾⠟⠛⢳⡼⢁⠋⠀⠀⠀⠀⠀
⠀⠰⣦⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⡏⠀⠀⠀⠓⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢿⠙⠻⢤⣀⣈⣷⡚⠛⠲⠦⣄⣨⣿⡟⢠⡿⠁⢀⣴⠟⢡⠎⠀⠀⠀⠀⠀⠀
⠀⠀⠹⡏⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠞⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣼⣧⡀⠀⠀⠀⠉⠉⠳⣄⠀⠈⠙⣿⢁⣿⣥⣶⡟⢁⣠⡤⠔⠒⠒⠒⠒⠀⠀
⠀⠀⠀⢻⣦⡀⣀⡀⠀⠀⠀⠀⠀⣠⡴⠛⠉⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣴⣿⣏⡁⢎⠻⠶⣦⣄⠀⠀⠀⠈⣧⠀⠀⣿⣼⡿⣿⣿⠟⠋⢁⣀⠀⠀⠀⠀⠀⠀⣀
⠀⠀⠀⠀⢿⡁⠉⠉⠉⠁⢀⣴⠞⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣴⣟⣛⡛⠉⣿⡻⢷⣶⣾⡶⣿⢷⡄⠀⠀⢸⡷⣦⣸⣿⣿⣶⣿⣶⣾⣿⣦⠀⢀⣀⡤⠖⠋⠁
⠀⠀⠀⠀⠈⢷⡀⠀⠀⢠⣿⡅⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣴⣾⣿⣿⣿⣿⣿⣶⣿⣷⣄⠙⢯⡉⠙⠛⠛⢦⣄⡋⢛⠀⢻⠋⣠⡟⢸⣿⣋⣀⣼⣶⡟⢥⡖⠋⠁⠀
⠀⠀⠀⠀⠀⠈⡇⠀⠀⣿⠋⢻⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣴⡾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣦⡀⠙⠒⠒⠒⠂⠻⣿⣺⠀⣼⠟⠉⢀⣼⣿⣿⣿⣿⣙⢿⡌⢷⡀⠀⠀
⠀⠀⠀⠀⠀⠀⣇⠀⠀⢸⣦⣌⣻⣷⣤⣄⣀⠀⠀⣀⣤⣶⣾⣿⣿⣿⣶⣿⣿⣿⣿⣿⣿⣿⣽⣿⣿⣏⣹⣿⣦⡀⠀⢀⣤⡴⠛⢿⡞⠃⠀⢠⣾⣧⣅⡀⠸⣿⣿⣾⢳⡄⢳⣄⠀
⠀⠀⠀⠀⠀⠀⠙⠷⣤⣄⣉⠻⠿⣶⣤⣭⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠇⢻⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣾⣿⠏⢀⡴⠿⠿⢶⣾⣋⠁⠀⠈⢳⣄⣿⣿⣿⣧⣿⡆⢸⢠
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢨⡿⣠⣿ ⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⠀⠈⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⢁⡴⠛⠃⣀⠀⠀⠙⡇⢀⣀⣀⣈⣿⣿⣿⣿⣿⣿⠇⣼⣻
⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠞⢰⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠗⠶⠦⢼⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣶⣾⣿⣿⣿⣿⣿⣷⣤⣿⣿⣿⣿⣿⣿⣿⣿⣿⠟⢁⡼⣹⣿
⣤⣤⡀⠀⠀⢀⣤⣴⠞⠋⠀⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡏⠀⠀⠀⠀⠺⣻⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⠋⠁⠀⠈⣸⣿⣿
⣿⣿⣿⣷⠞⢉⡽⠁⠀⠀⢰⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⣿⣿⣿⠏⠀⢀⣠⣤⣤⣀⠉⢪⠻⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡅⠀⠀⠀⠀⣿⣿⣿
⣿⠿⠋⠀⢀⠞⠀⠀⠀⢠⣾⣿⣿⣿⣿⣿⣿⠟⣹⣥⡀⣰⣿⡿⠁⠀⠀⢩⣾⣿⣿⡙⠿⣦⣍⠈⠛⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡇⠀⠀⠀⠀⢻⡿⢸
⠀⠀⠀⢠⠏⠀⠠⠤⠾⠿⠛⣿⣿⣿⣿⣿⣿⣰⣟⣿⣾⠟⠉⠀⠀⠀⠀⠀⣿⣟⣿⡟⠓⠺⢿⣿⠦⠞⣿⣿⣿⣿⣿⣿⣿⡟⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠃⠀⠀⠀⠀⠈⠃⠘
⠀⠀⢠⠏⠀⠀⠀⣠⠶⠆⢸⡿⢻⣿⣿⣿⣿⣿⡟⣿⣟⠀⠀⠀⠀⠀⠀⠀⠹⣿⣻⡟⣀⡀⠚⠁⠀⠀⣿⣿⣿⣿⣿⣿⣯⣤⡈⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠀⠀⠀⠀⠀⢀⣀⠀
⠀⢠⠏⠀⠀⣠⠞⠁⠀⠀⢸⡇⠀⢻⣿⣿⣿⣧⠁⣻⣿⣷⠀⠀⠀⠀⠀⠀⠀⠈⠉⠉⠉⠀⠀⠀⠀⢀⣿⠃⢹⣿⣿⣿⣡⣾⡇⢸⣿⣿⣿⣿⣿⣿⣿⡿⣿⣧⡀⠀⠀⠀⣾⣿⣿
⢀⠏⢀⡠⠞⠉⠀⠀⠀⠀⠘⠃⠁⢸⣿⣿⣿⣿⠀⠉⣹⠋⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡾⣁⢠⣾⣿⣿⣿⣿⠛⣰⣿⣿⣿⣿⣿⣿⣿⣿⠁⠈⠉⠛⠒⢀⣴⣿⣿⣿
⢞⠀⠈⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⠀⠈⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠚⣱⣿⣿⣿⣿⣿⣿⣾⣿⣿⣿⣿⣿⣿⠟⣿⠁⠀⠀⠀⠀⠁⣸⣿⣿⣿⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡾⠿⠛⣿⣿⣿⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣰⠟⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⢃⣤⣘⠇⠀⠀⣰⣿⣿⣿⣿⣿⣿⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠹⣿⣿⣿⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡴⠋⠀⠹⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡇⢸⣿⠙⠛⠛⠻⠛⣀⣽⡿⣿⢿⣿⠿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣿⣿⣿⠙⢧⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣤⠟⠀⠀⣠⣾⣿⣿⣿⡟⠙⣿⣿⣿⠻⣿⣷⠸⠿⠶⠶⠶⣦⣄⠁⠂⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣿⡟⣿⣿⣧⠀⠻⣄⠀⠀⠀⠀⠀⠀⠀⢀⣠⡴⠏⠁⠀⠀⢀⢿⣿⣿⣿⣿⠇⠀⢹⣿⢿⣶⣾⣿⡶⠖⠒⠒⠲⣆⢻⣆⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⣿⣿⣿⠁⠀⠀⠘⣦⡀⠀⢀⣀⣴⣾⠿⣅⠀⠀⠀⠀⠀⠀⣸⣿⣿⣟⠁⢀⣀⣠⣿⡿⠟⠋⠁⠀⠀⠀⠀⠀⠹⣆⠻⣦⡀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣴⠟⢻⣿⢻⡄⠀⠀⠀⠈⣙⣛⣻⣭⣴⠶⡛⢿⡆⠀⠀⠀⢀⣼⡿⢿⣿⣿⣟⣿⡿⠋⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠳⣬⡙⠂⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣧⣸⣧⣿⡥⠖⠒⠋⠉⠉⣡⣾⠏⣡⣤⣯⣿⣿⣤⣶⣾⡿⣿⣿⣞⣿⣿⣿⠏⠀⠀⠀⢀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⡈⠙⠓⠲⢦⠀
⠀⠀⠀⠀⠀⠀⠀⠘⠶⣶⢤⠤⠤⠤⣶⣾⢿⣿⡇⠀⠀⠀⠀⢀⣾⠟⠀⠀⣿⣿⠿⢿⡿⠚⠋⢹⠀⣿⡇⣿⡟⢉⣟⣠⣤⣶⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣯⣭⣉⣀⠀⠈⢠
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠙⠓⠲⠾⢿⣀⣼⣿⠧⣶⣶⣶⣿⣿⣿⡀⠀⢠⣿⡏⠀⢸⡇⠀⠀⢸⡆⢻⣿⣿⣷⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡷⠆⣼
⠀⠀⠀⠀⠀⠀⠀⠀⢀⠀⠀⠀⣠⣄⡀⣹⠟⣠⡾⠛⣻⣿⣿⣿⣿⣿⣀⣼⠏⠀⠀⣾⠀⠀⠀⢸⣇⣼⣿⣿⣯⠄⡌⢻⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣾⣿⣿⣿⣿⡿⣿⠃⣼⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠸⣧⣀⣠⣿⡙⣛⣿⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡏⠀⠀⣼⣧⣤⣶⣿⣿⣿⣿⣿⣿⡇⣀⣰⣞⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠃⣸⣿⣿
*/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvl vector<vector<ll>>
#define el '\n'
#define pb push_back
#define mp make_pair
#define pii pair<int,int>
#define fi first
#define se second
#define all(v) v.begin(), v.end()
template<class T> using pq = priority_queue<T>;
template<class T> bool ckmin(T& a,const T& b) {return b<a ? a=b,1:0;}
template<class T> bool ckmax(T& a,const T& b) {return a<b ? a=b,1:0;}
template<typename T, typename V>
void __print(const pair<T, V> &x);
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#define sz(x) (int)(x).size()
#define lb lower_bound
#define ub upper_bound
const int mod=1e9+7;
//const int mod=998244353;
void solve(){
int n,m;cin>>n>>m;
vi b(m),p(m);
int target=-1;
for(int i=0;i<m;i++){
cin>>b[i]>>p[i];
if(i==1)target=b[i];
}
unordered_map<int,vi>init;
for(int i=0;i<m;i++)init[b[i]].pb(p[i]);
unordered_map<int,unordered_set<int>> dynamic;
priority_queue<tuple<int, int, int>,vector<tuple<int, int, int>>,greater<tuple<int,int,int>>> pq;
unordered_map<int, unordered_map<int,int>> dist;
int startx=b[0];
int startp=p[0];
pq.emplace(0,startx,startp);
dist[startx][startp]=0;
int ans=-1;
while(!pq.empty()){
auto[cost, x, p]=pq.top();pq.pop();
if(x==target){
ans=cost;
break;
}
if (dist[x].count(p)&&dist[x][p]<cost) continue;
dynamic[x].insert(p);
for(int delta:{p,-p}){
int new_x = x+delta;
if (new_x<0 or new_x>=n)continue;
int new_cost=cost+1;
if(!dist[new_x].count(p) or new_cost<dist[new_x][p]){
dist[new_x][p]=new_cost;
pq.emplace(new_cost,new_x,p);
}
}
vi avail=init[x];
for(int pp:dynamic[x])avail.pb(pp);
for(int p_prime:avail){
if(p_prime==p)continue;
if(!dist[x].count(p_prime) or cost<dist[x][p_prime]){
dist[x][p_prime]=cost;
pq.emplace(cost,x,p_prime);
}
}
}
cout<<ans<<el;
}
int main(){
cin.tie(0)->sync_with_stdio(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
solve();
return 0;
}
