/*
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);
int t = 1;
//cin >> t;
while (t--) solve();
return 0;
}
vector<int> memo, b, p;
int n, m;
void solve() {
cin >> n >> m;
b.clear(); p.clear(); memo.clear();
b.resize(m); p.resize(m); memo.resize(n + 1, -1);
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
}
if (b[0] == b[1]) {
cout << "0\n";
return;
}
priority_queue<pair<int,int>, vector<pair<int,int>>, greater<pair<int,int>>> q;
q.push({0,0});
vector<int> dist(m, inf); dist[0] = 0;
vector<bool> processed(m, false);
while(!q.empty()) {
auto [u,v] = q.top(); q.pop();
if (processed[v]) continue;
processed[v] = true;
for (int i = 0; i < m; i++) {
if (i == v) continue;
if (abs(b[i] - b[v]) % p[v] == 0) {
if (dist[i] > dist[v] + abs(b[i] - b[v]) / p[v]) {
dist[i] = dist[v] + abs(b[i] - b[v]) / p[v];
q.push({dist[i], i});
}
}
}
}
cout << (dist[1] == inf ? -1 : dist[1]) << '\n';
}
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