#include <bits/stdc++.h>
using namespace std;
#define pii pair<int,int>
#define pb push_back
#define endl '\n'
#define fast_io ios_base::sync_with_stdio(0) ,cin.tie(0),cout.tie(0);
#pragma GCC optimize("O3,unroll-loops")
const int maxn = 3e4+4 , mod = 1e9+7 , inf = 1e7;
int n,m;
int p[maxn];
int b[maxn];
int dis[2003];
priority_queue <pii , vector<pii> , greater<pii>> pq;
int32_t main(){
    fast_io
    cin >> n >> m;
    vector <vector<int16_t>> vec(n);
    for(int i = 0 ; i < n ; i++) dis[i] = inf;
    for(int i = 0 ; i < m ; i++){
        cin >> b[i] >> p[i];
        vec[b[i]].push_back(i);
    }
    dis[b[0]] = 0 , pq.push({0,b[0]});
    while(pq.size()){
        auto [d,v] = pq.top();
        pq.pop();
        if(d > dis[v])continue;
        vector <pii> adj;
        for(auto i : vec[v]){
            for(int j = 0 ; j < n ; j++){
                if(j%p[i] == b[i]%p[i]) adj.pb({j,abs(j-b[i])/p[i]});
            }
        }
        for(auto [u,w] : adj){
            if(dis[v]+w < dis[u]){
                dis[u] = dis[v]+w;
                pq.push({dis[u],u});
            }
        }
    }
    if(dis[b[1]] != inf) cout << dis[b[1]] << endl;
    else cout << -1 << endl;
    return 0;
}
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