#include <bits/stdc++.h>
#define ff first
#define ss second
#define mp make_pair
#define pb push_back
#define pp push
#define MOD 1000000007
#define INF 1e18
#define N 30005
#define M 5000000
const int c=174;
typedef long long ll;
using namespace std;
int b[N],p[N],pos[N],path[N];
bool vis[N];
set<int>v[c][c];
int main(){
// ios_base::sync_with_stdio(0);
// cin.tie(0);
// cout.tie(0)
memset(pos,-1,sizeof(pos));
memset(path,-1,sizeof(path));
int n,m;
cin >> n >> m;
for(int i=0;i<m;i++){
cin >> b[i] >> p[i];
pos[b[i]]=i;
for(int j=1;j<c;j++){
v[j][b[i]%j].insert(i);
}
}
priority_queue<pair<int,int> >pq;
pq.push({0,0});
while(!pq.empty()){
int cost=-pq.top().ff;
int doge_id=pq.top().ss;
path[doge_id]=cost;
pq.pop();
vis[doge_id]=1;
if(p[doge_id]>=c){
for(int i=b[doge_id]%p[doge_id];i<n;i+=p[doge_id]){
if(pos[i]!=-1){
if(vis[pos[i]]==0){
pq.push({-(cost+(abs(b[doge_id]-i))/p[doge_id]),pos[i]});
}
}
}
}
else{
for(int i=1;i<c;i++){
v[i][b[doge_id]%i].erase(doge_id);
}
for(auto it=v[p[doge_id]][b[doge_id]%p[doge_id]].begin();it!=v[p[doge_id]][b[doge_id]%p[doge_id]].end();++it){
if(vis[*it]==0){
pq.push({-(cost+(abs(b[*it]-b[doge_id]))/p[doge_id]),*it});
}
}
}
while(!pq.empty() && vis[pq.top().ss]==1) pq.pop();
}
cout << path[1];
}
