/*
AUTHOR : BELGIANBOT
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template<typename T> void read(T& x) { cin >> x;}
template<typename T1, typename T2> void read(pair<T1, T2>& p) { read(p.first), read(p.second);}
template<typename T> void read(vector<T>& v) { for (auto& x : v) read(x); }
template<typename T1, typename T2> void read(T1& x, T2& y) { read(x), read(y); }
template<typename T1, typename T2, typename T3> void read(T1& x, T2& y, T3& z) { read(x), read(y), read(z); }
template<typename T1, typename T2, typename T3, typename T4> void read(T1& x, T2& y, T3& z, T4& zz) { read(x), read(y), read(z), read(zz); }
template<typename T> void print(vector<T>& v) { for (auto& x : v) cout << x << ' '; cout << endl; }
#define pb push_back
#define fi first
#define se second
#define ordered_set tree<pair<int,int>, null_type, less<pair<int,int>>, rb_tree_tag, tree_order_statistics_node_update>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define mp make_pair
#define sz(x) (int)(x.size())
#define YES cout<<"YES\n";
#define Yes cout<<"Yes\n";
#define NO cout<<"NO\n";
#define No cout << "No\n";
#define cmin(a, b) a = min(a, b)
#define cmax(a, b) a = max(a, b)
//#define int long long
#define double long double
const int inf = numeric_limits<int>::max() / 4;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int MOD = 1e9+7, LOG=20;
void solve();
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
int t = 1;
//cin >> t;
while (t--) solve();
return 0;
}
vector<set<int>> line;
int n, m;
void solve() {
cin >> n >> m;
line.clear();
line.resize(n);
int start, end;
for (int i = 0; i < m; i++) {
int a, b; cin >> a >> b;
line[a].insert(b);
if (!i) start = a;
if (i == 1) end = a;
}
queue<vector<int>> q;
q.push({start, 0, 0});
int ans = -1;
while(!q.empty()) {
vector<int> u = q.front(); q.pop();
int x = u[0], res = u[2], jump = u[1];
if (x == end || res > 2*n) {
ans = res;
break;
}
for (int y : line[x]) {
if (y == abs(jump)) continue;
if (x - y >= 0) q.push({x - y, -y, res + 1});
if (x + y < n) q.push({x + y, y, res + 1});
}
if (x + jump >= 0 && x + jump < n) q.push({x + jump, jump, res + 1});
}
cout << (ans > 2*n ? -1 : ans) << '\n';
}
