#include<bits/stdc++.h>
#define pb push_back
#define int long long
#define mp make_pair
using namespace std;
const int inf = 1e18;
const int mod = 1e9 + 7;
const int N = 2e3 + 5;
vector<vector<int> > dis(N, vector<int>(N, inf));
int32_t main(){
ios_base::sync_with_stdio(false), cin.tie();
int n, m;
cin>>n>>m;
int start = 0, finish = 0;
for(int i = 0; i < m; i++){
int b, p;
cin>>b>>p;
if(i == 0) start = b;
if(i == 1) finish = b;
for(int k = b%p; k < n; k += p){
dis[b][k] = min(dis[b][k], abs(b - k)/p);
}
}
vector<pair<int, int> > adj[N];
for(int i = 0; i < n; i++){
for(int j = 0; j < n; j++){
if(i == j || dis[i][j] == inf) continue;
adj[i].pb(mp(j, dis[i][j]));
}
}
priority_queue<pair<int, int> > pq;
vector<int> best(N, inf);
best[start] = 0;
pq.push(mp(0, start));
while(pq.size()){
int node = pq.top().second;
int len = pq.top().first * (-1);
pq.pop();
best[node] = len;
for(auto itr: adj[node]){
if(best[itr.first] > best[node] + itr.second){
pq.push(mp(-(best[node] + itr.second), itr.first));
}
}
}
if(best[finish] == inf) cout<<-1<<endl;
else cout<<best[finish]<<endl;
return 0;
}
