#include <bits/stdc++.h>
// #pragma GCC optimize ("Ofast,unroll-loops")
// #pragma GCC target ("avx2")
using namespace std;
typedef long long ll;
typedef pair<int, int> pp;
#define per(i,r,l) for(int i = (r); i >= (l); i--)
#define rep(i,l,r) for(int i = (l); i < (r); i++)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define pb push_back
#define ss second
#define ff first
void err(istringstream *iss){}template<typename T,typename ...Args> void err(istringstream *iss,const T &_val, const Args&...args){string _name;*iss>>_name;if(_name.back()==',')_name.pop_back();cerr<<_name<<" = "<<_val<<", ",err(iss,args...);}
void IOS(){
cin.tie(0) -> sync_with_stdio(0);
#ifndef ONLINE_JUDGE
#define er(args ...) cerr << __LINE__ << ": ", err(new istringstream(string(#args)), args), cerr << endl
#else
#define er(args ...) 0
#endif
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const ll mod = 1e9 + 7, maxn = 3e4 + 5, lg = 22, inf = ll(1e9) + 5;
ll pw(ll a,ll b,ll md=mod){if(!b)return 1;ll k=pw(a,b>>1ll,md);return k*k%md*(b&1ll?a:1)%md;}
vector<int> dog[maxn];
map<pp, int> dist;
bool vis[maxn];
int main(){ IOS();
int n, m; cin >> n >> m;
map<pp, vector<int>> is;
pp bg, en;
rep(i,0,m){
int x, p; cin >> x >> p;
if(!i) bg = {x, p};
if(i == 1) en = {x, p};
dog[x].pb(p);
}
deque<pp> q;
q.pb({bg});
dist[bg] = 0;
while(sz(q)){
auto[x, p] = q.front(); q.pop_front();
auto add = [&](int x2, int p2, bool bk){
if(x2 > -1 && x2 < n && !dist.count({x2, p2})){
dist[{x2, p2}] = dist[{x, p}] + bk;
if(bk) q.pb({x2, p2});
else q.push_front({x2, p2});
}
};
if(!vis[x]){
for(int r: dog[x]){
add(x, r, false);
}
}
vis[x] = true;
add(x+p, p, true), add(x-p, p, true);
}
cout << (dist.count(en)? dist[en]: -1) << '\n';
return 0-0;
}
