/*
Code written by Talant I.D.
*/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;
#define precision(n) fixed << setprecision(n)
#define pb push_back
#define ub upper_bound
#define lb lower_bound
#define mp make_pair
#define eps (double)1e-9
#define PI 2*acos(0.0)
#define endl "\n"
#define sz(v) int((v).size())
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define do_not_disturb ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define OK cout << "OK" << endl;
const int mod = 998244353;
ll mode(ll a) {
a %= mod;
if (a < 0) a += mod;
return a;
}
ll subt(ll a, ll b) {
return mode(mode(a)-mode(b));
}
ll add(ll a, ll b) {
return mode(mode(a)+mode(b));
}
ll mult(ll a, ll b) {
return mode(mode(a)*mode(b));
}
ll binpow(ll a, ll b) {
ll res = 1;
while (b) {
if (b&1) res = mult(res, a);
a = mult(a, a);
b >>= 1;
}
return res;
}
const int N = 3e4+7;
bitset <N> used, vis[N];
vector <int> powers[N];
int main() {
do_not_disturb
int n, m;
cin >> n >> m;
int b[m], p[m];
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
powers[b[i]].pb(p[i]);
}
deque <array<int, 3>> K;
K.pb({b[0], p[0], 0});
while (!K.empty()) {
auto v = K.front(); K.pop_front();
vis[v[0]][v[1]] = 1;
if (v[0] == b[1]) {
cout << v[2] << endl;
return 0;
}
if (!used[v[0]]) {
for (auto to : powers[v[0]]) {
if (vis[v[0]][to]) continue;
vis[v[0]][to] = 1;
K.push_front({v[0], to, v[2]});
}
}
used[v[0]] = 1;
if (v[0]+v[1] < n && vis[v[0]+v[1]][v[1]] == 0) {
K.pb({v[0]+v[1], v[1], v[2]+1});
}
if (v[0]-v[1] >= 0 && vis[v[0]-v[1]][v[1]] == 0) {
K.pb({v[0]-v[1], v[1], v[2]+1});
}
}
cout << -1;
return 0;
}
