#include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
#include<string>
#include<chrono>
#include<functional>
#include<iterator>
#include<random>
#include<unordered_set>
#include<array>
#include<map>
#include<iomanip>
#include<assert.h>
using namespace std;
typedef long long int llint;
typedef long double lldo;
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase
//cout<<setprecision(20)
//cin.tie(0);
//ios::sync_with_stdio(false);
const llint mod=1000000007;
const llint big=4.19e18+1;
const long double pai=3.141592653589793238462643383279502884197;
const long double eps=1e-15;
template <class T,class U>void mineq(T& a,U b){if(a>b){a=b;}}
template <class T,class U>void maxeq(T& a,U b){if(a<b){a=b;}}
llint gcd(llint a,llint b){if(a%b==0){return b;}else return gcd(b,a%b);}
llint lcm(llint a,llint b){return a/gcd(a,b)*b;}
template<class T> void SO(T& ve){sort(ve.begin(),ve.end());}
template<class T> void REV(T& ve){reverse(ve.begin(),ve.end());}
int LBI(vector<int>&ar,llint in){return lower_bound(ar.begin(),ar.end(),in)-ar.begin();}
int UBI(vector<int>&ar,llint in){return upper_bound(ar.begin(),ar.end(),in)-ar.begin();}
int main(void){
//0-1 dfs
int n,m,i;cin>>n>>m;
vector<vector<int>>go(n);
int stb,stp,enb;
for(i=0;i<m;i++){
int b,p;cin>>b>>p;
if(i==0){stb=b;stp=p;}
if(i==1){enb=b;}
go[b].pub(p);
}
vector<unordered_set<int>>used(n);
deque<tuple<int,int,int,bool>>que;
que.pub(mt(stb,stp,0,true));
que.pub(mt(stb,stp,0,false));
used[stb].ins(stp);
while(que.size()>0){
int b,p,t;bool mu;tie(b,p,t,mu)=que.front();
que.pof();
if(b==enb){cout<<t<<endl;return 0;}
//cerr<<"de"<<b<<" "<<p<<" "<<t<<endl;
used[b].ins(p);
if(go[b].size()>0){
for(auto it:go[b]){que.puf(mt(b,it,t,true));que.puf(mt(b,it,t,false));}
go[b].clear();
}
if(mu&&b+p<n&&used[b+p].count(p)==0){
que.pub(mt(b+p,p,t+1,true));
used[b+p].ins(p);
}
if((!mu)&&b-p>=0&&used[b-p].count(p)==0){
que.pub(mt(b-p,p,t+1,false));
used[b-p].ins(p);
}
}
cout<<"-1"<<endl;
return 0;
}
