This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
///*                                                    __                    __                        __                    */
///*        ======     _      /| /|  __   _            /   |  |   /|  |   *  |    |  |  | /   /| |\  | /   |  |  * | /        */
///* \-       ||  |_| |_     / |/ | |  | |_  |-        |   |--|  /-|  |   |  \ \  |==|  |-   /=| | \ | |   |--|  | |-         */
///*          ||  | | |_    /     | |__|  _| |_        \__ |  | /  |  |__ |  __|  |  |  | \ /  | |  \| \__ |  |  | | \        */
///* 
// autor :: Rick Prime
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define ff first
#define ss second
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define int long long
#define Mp make_pair
#define double double long
typedef vector<int> vi;
typedef vector<double> vd;
typedef pair<int,int> pii;
typedef vector<pii> vii;
const int N = 1e6, inf = 2e18, mod = 1e9+7;
void solve(){
	int n, m;
	cin >> n >> m;
	vii v(m);
	for(auto &[i, j]:v)cin >> i >> j;
	int dp[n];
	for(int &i:dp)i = inf;
	dp[v[0].ff] = 0;
	vi r[n];
	for(auto [i, j]:v)r[i].pb(j);
	priority_queue<pii, vii, greater<pii>> q;
	q.push({0, v[0].ff});
	while(!q.empty()){
		auto [dist, b] = q.top();
		q.pop();
		if(dp[b] < dist)continue;
		
		for(auto x:r[b]){
			int cnt = 0;
            for (int i = b; i >= 0; i -= x) {
                if (dp[i] > dist + cnt) {
                    dp[i] = dist + cnt;
                    q.push({dp[i], i});
                }
                cnt++;
            }
 
            cnt = 0;
            for (int i = b; i < n; i += x) {
                if (dp[i] > dist + cnt) {
                    dp[i] = dist + cnt;
                    q.push({dp[i], i});
                }
                cnt++;
            }
		}
	}
	//for(auto i:dp)cout << i << ' ';
	//cout << '\n';
	cout << (dp[v[1].ff]==inf?-1:dp[v[1].ff]) << '\n';
}
 
signed main(){
	ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
	int test = 1;
	//cin >> test;
	while(test--){
		solve();
	}
}
 
 
 
 
 
 
 
 
 
 
 
 
 
| # | Verdict  | Execution time | Memory | Grader output | 
|---|
| Fetching results... | 
| # | Verdict  | Execution time | Memory | Grader output | 
|---|
| Fetching results... | 
| # | Verdict  | Execution time | Memory | Grader output | 
|---|
| Fetching results... | 
| # | Verdict  | Execution time | Memory | Grader output | 
|---|
| Fetching results... | 
| # | Verdict  | Execution time | Memory | Grader output | 
|---|
| Fetching results... |