#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
#define fi first
#define se second
#define mp make_pair
#define fastIO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const int N = (int)3e4 + 10;
map<pii, int> dist;
bool has[N];
vector<int> dj[N];
int main(){
fastIO;
int n, m;
cin >> n >> m;
deque<pii> ff;
int xi, pi;
int goalx = 0;
for(int i = 0 ; i < m ; i ++ ){
cin >> xi >> pi;
if(i == 0){
if(!dist.count(mp(xi, pi))){
dist[mp(xi, pi)] = 0;
ff.push_back(mp(xi, pi));
}
}
if(i == 1){
goalx = xi;
}
dj[xi].push_back(pi);
}
int node, dd;
int cur;
while(!ff.empty()){
node = ff.front().fi;
dd = ff.front().se;
ff.pop_front();
cur = dist[mp(node, dd)];
if(node == goalx){
cout << cur << "\n";
return 0;
}
if(!has[node]){
for(auto x : dj[node]){
if(!dist.count(mp(node, x))){
dist[mp(node, x)] = cur;
ff.push_front(mp(node, x));
}
}
has[node]=true;
}
if(node - dd > 0){
if(!dist.count(mp(node - dd, dd))){
dist[mp(node - dd, dd)] = cur + 1;
ff.push_back(mp(node - dd, dd));
}
}
if(node + dd < n){
if(!dist.count(mp(node + dd, dd))){
dist[mp(node + dd, dd)] = cur + 1;
ff.push_back(mp(node + dd, dd));
}
}
}
cout << "-1\n";
return 0;
}
