// Om Namah Shivaya
// GM in 94 days
#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 <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long int ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL)
#define endl '\n'
#define pb push_back
#define conts continue
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define yes cout << "YES" << endl
#define no cout << "NO" << endl
#define ff first
#define ss second
#define ceil2(x,y) ((x+y-1) / (y))
#define sz(a) a.size()
#define setbits(x) __builtin_popcountll(x)
#ifndef ONLINE_JUDGE
#define debug(x) cout << #x <<" = "; print(x); cout << endl
#else
#define debug(x)
#endif
#define rep(i,n) for(int i = 0; i < n; ++i)
#define rep1(i,n) for(int i = 1; i <= n; ++i)
#define rev(i,s,e) for(int i = s; i >= e; --i)
#define trav(i,a) for(auto &i : a)
bool iseven(ll n) {if ((n & 1) == 0) return true; return false;}
void print(ll t) {cout << t;}
void print(int t) {cout << t;}
void print(string t) {cout << t;}
void print(char t) {cout << t;}
void print(double t) {cout << t;}
void print(ld t) {cout << t;}
template <class T, class V> void print(pair <T, V> p);
template <class T> void print(vector <T> v);
template <class T> void print(set <T> v);
template <class T, class V> void print(map <T, V> v);
template <class T> void print(multiset <T> v);
template <class T, class V> void print(pair <T, V> p) {cout << "{"; print(p.ff); cout << ","; print(p.ss); cout << "}";}
template <class T> void print(vector <T> v) {cout << "[ "; for (T i : v) {print(i); cout << " ";} cout << "]";}
template <class T> void print(set <T> v) {cout << "[ "; for (T i : v) {print(i); cout << " ";} cout << "]";}
template <class T> void print(multiset <T> v) {cout << "[ "; for (T i : v) {print(i); cout << " ";} cout << "]";}
template <class T, class V> void print(map <T, V> v) {cout << "[ "; for (auto i : v) {print(i); cout << " ";} cout << "]";}
template<typename T> void amin(T &a, T b) { a = min(a, b); }
template<typename T> void amax(T &a, T b) { a = max(a, b); }
void usaco(string filename) {
freopen((filename + ".in").c_str(), "r", stdin);
freopen((filename + ".out").c_str(), "w", stdout);
}
const int MOD = 1e9 + 7;
const int maxn = 3e4 + 5;
const int inf1 = 1e9 + 5;
const ll inf2 = ll(1e18) + 5;
const int block_size = 175;
vector<array<int, 3>> adj[maxn][block_size];
bool vis[maxn][block_size];
int dis[maxn][block_size];
void solve(int test_case)
{
// editorial approach
int n, m; cin >> n >> m;
vector<pii> a(m);
rep(i, m) {
int b, p; cin >> b >> p;
a[i] = {b, p};
if (p < block_size) {
adj[b][0].pb({b, p, 0});
}
else {
int w = 0;
for (int j = b - p; j >= 0; j -= p) {
w++;
adj[b][0].pb({j, 0, w});
}
w = 0;
for (int j = b + p; j < n; j += p) {
w++;
adj[b][0].pb({j, 0, w});
}
}
}
priority_queue< array<int, 3>, vector<array<int, 3>>, greater<array<int, 3>> > pq;
rep(i, n) rep(j, block_size) dis[i][j] = inf1;
if (a[0].ss < block_size) {
pq.push({0, a[0].ff, a[0].ss});
dis[a[0].ff][a[0].ss] = 0;
}
else {
pq.push({0, a[0].ff, 0});
dis[a[0].ff][0] = 0;
}
auto add = [&](int cost, int b, int p) {
if (cost < dis[b][p]) {
dis[b][p] = cost;
pq.push({cost, b, p});
}
};
while (!pq.empty()) {
auto [cost, b, p] = pq.top();
pq.pop();
if (vis[b][p]) conts;
vis[b][p] = 1;
if (b == a[1].ff) {
cout << cost << endl;
return;
}
// add edge to dummy vertex of b
add(cost, b, 0);
// go 1 step left / 1 step right
if (p > 0) {
if (b - p >= 0) add(cost + 1, b - p, p);
if (b + p < n) add(cost + 1, b + p, p);
}
// traverse other edges and add them
for (auto [b2, p2, w] : adj[b][p]) {
add(cost + w, b2, p2);
}
}
cout << -1 << endl;
}
int main()
{
chrono::steady_clock::time_point begin = chrono::steady_clock::now();
fastio;
int t = 1;
// cin >> t;
rep1(i, t) {
solve(i);
}
chrono::steady_clock::time_point end = chrono::steady_clock::now();
cerr << "[Finished in " << chrono::duration_cast<chrono::microseconds>(end - begin).count() / 1000 << "ms]" << endl;
return 0;
}
