#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define pb(e) push_back(e)
#define sv(a) sort(a.begin(),a.end())
#define sa(a,n) sort(a,a+n)
#define mp(a,b) make_pair(a,b)
#define vf first
#define vs second
#define ar array
#define all(x) x.begin(),x.end()
const int inf = 0x3f3f3f3f;
const int mod = 998244353;
const double PI=3.14159265358979323846264338327950288419716939937510582097494459230;
bool remender(ll a , ll b){return a%b;}
//freopen("problemname.in", "r", stdin);
//freopen("problemname.out", "w", stdout);
void solve(){
int n , m1;
cin >> n >> m1;
vector<vector<int>> v(n);
map<pair<int,int>,int> m;
int dis[n];
memset(dis,0,sizeof dis);
int s , e;
for(int i = 0; i < m1; i++){
int b,p;
cin >> b >> p;
if(i == 0)s = b;
v[b].pb(p);
if(i == 1)e = b;
}
queue<array<int,3>> q;
memset(dis,-1,sizeof dis);
dis[s] = 0;
for(int i : v[s]){
if(m[{s,i}] == 0){
array<int,3> lol = {s,i,0};
q.push(lol);
}
}
while(q.size()){
array<int,3> p = q.front();
q.pop();
if(p[0] + p[1] < n && m[{p[0]+p[1] , p[1]}] == 0){
m[{p[0]+p[1] , p[1]}] = 1;
array<int,3> lol = {p[0]+p[1],p[1],p[2]+1};
q.push(lol);
if(dis[p[0]+p[1]] == -1){
dis[p[0]+p[1]] = p[2]+1;
for(int i : v[p[0]+p[1]]){
if(m[{p[0]+p[1],i}]==0){
array<int,3> again = {p[0]+p[1] , i , dis[p[0]+p[1]]};
q.push(again);
}
}
}
}
if(p[0] - p[1] >=0 && m[{p[0]-p[1] , p[1]}] == 0){
m[{p[0]-p[1] , p[1]}] = 1;
array<int,3> lol = {p[0]-p[1],p[1],p[2]+1};
q.push(lol);
if(dis[p[0]-p[1]] == -1){
dis[p[0]-p[1]] = p[2]+1;
for(int i : v[p[0]-p[1]]){
if(m[{p[0]-p[1],i}] == 0){
array<int,3> again = {p[0]-p[1] , i , dis[p[0]-p[1]]};
q.push(again);
}
}
}
}
}
cout << dis[e] << '\n';
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//int t;cin >> t;while(t--)
solve();
return 0;
}
