// #pragma GCC optimize("Ofast")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
// #pragma GCC optimize("unroll-loops")
#include "bits/stdc++.h"
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/trie_policy.hpp>
// #include <ext/rope>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
gp_hash_table<int, int> table;
using namespace std;
// using namespace __gnu_pbds;
// using namespace __gnu_cxx;
#define li long long
#define ld long double
#define ii pair<int, int>
#define vi vector<int>
#define vvi vector<vi>
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pf push_front
#define eb emplace_back
#define em emplace
#define ob pop_back
#define om pop
#define of pop_front
#define fr front
#define bc back
#define fori(i, a, b) for(int i = (int) (a); i <= (int) (b); ++i)
#define ford(i, a, b) for(int i = (int) (a); i >= (int) (b); --i)
#define all(x) begin(x), end(x)
#define sz(x) ((int)(x).size())
#define bitc __builtin_popcountll
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//#define rand rng
#define endl '\n'
#define sp ' '
#define fastio() ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
void solve();
signed main() {
// freopen("input.inp","r",stdin);
// freopen("output.out","w",stdout);
fastio();
int tc = 1;
fori(test, 1, tc) {
solve();
}
return 0;
}
///#define int short
//long long
const ld pi = 4 * atan(1.0), eps = 1e-9;
const int maxn = 30000 + 5;
//using namespace __gnu_pbds;
//gp_hash_table<int, int> table;
//const int RANDOM = chrono::high_resolution_clock::now().time_since_epoch().count();
//struct chash {
// int operator()(int x) const { return x ^ RANDOM; }
//};
//gp_hash_table<key, int, chash> table;
gp_hash_table<int, int> dis[maxn];
int n, m;
int p[maxn];
int b[maxn];
vector<int> S[maxn];
const int bl = 2000;
int disarray[maxn][bl];
const int inf = INT_MAX / 2;
int * distpt(int i, int j) {
if(j < bl) return &disarray[i][j];
auto it = dis[i].find(j);
if(it == dis[i].end() ) {
return &(dis[i][j] = inf);
}
return &(it->se);
}
void solve() {
cin >> n >> m;
fori(i, 0, m - 1) {
int b, p; cin >> b >> p;
::b[i] = b; ::p[i] = p;
S[b].eb(p);
}
fori(i, 0, n - 1) fori(j, 0, bl - 1) disarray[i][j] = inf;
disarray[b[0]][0] = 0;
deque<ii> q;
q.eb(b[0], 0ll);
while(!q.empty()) {
int i = q.front().fi, j = q.front().se;
q.of();
if(i == b[1]) {
cout << *distpt(i, j);
return;
}
#define ef emplace_front
if(j) {
auto nxt = distpt(i, 0), now = distpt(i, j);
// cout << *now << endl;
if(*nxt > *now) {
*nxt = *now;
q.ef(i, 0);
}
for(int ti: { i - j, i + j}) {
if(ti >= 0 and ti < n) {
nxt = distpt(ti, j);
if(*nxt > *now + 1) {
*nxt = *now + 1;
q.eb(ti, j);
}
}
}
}
else {
auto now = distpt(i, j);
for(int tj: S[i]) {
auto nxt = distpt(i, tj);
if(*nxt > *now) {
*nxt = *now;
q.ef(i, tj);
}
}
}
}
cout << -1;
}
