#include<bits/stdc++.h>
using namespace std;
#define INIT ios_base :: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define mp make_pair
#define pb push_back
#define ft first
#define sc second
#define ll long long
#define pii pair<int, int>
#define count_bits __builtin_popcount
int t, n, m, k, b[30010], p[30010];
int crd[30010];
int d[2010];
bool v[2010];
bool dog[2010];
vector<pii> g[30010];
void dijkstra(){
multiset<pii> ms;
ms.insert({d[crd[1] ], crd[1]});
while(!ms.empty()){
auto f=*ms.begin(); ms.erase(ms.begin());
int nod=f.sc;
v[nod]=true;
//cout<<nod<<"y\n";
if(nod==crd[2]);
for(auto pr:g[nod]){
int u=pr.ft, c=pr.sc;
if(v[u]){continue;}
//cout<<u<<" "<<c<<"x\n";
auto it=ms.find({d[u], u});
if(it!=ms.end()){
ms.erase(it);
d[u]=min(d[u], d[nod]+c);
ms.insert({d[u], u});
}
else{
d[u]=d[nod]+c;
ms.insert({d[u], u} );
}
}
}
return;
}
int32_t main(){
INIT
cin>>n>>m;
for(int i=1; i<=n; i++){d[i]=-1;}
for(int i=1; i<=m; i++){
cin>>b[i]>>p[i]; b[i]++;
dog[b[i] ]=true;
}
for(int i=1; i<=m; i++){
for(int j=b[i]-p[i]; j>=1; j-=p[i]){
if(dog[j]){
// g[j].pb({b[i], abs(b[i]-j)/p[i] });
g[b[i] ].pb({j, abs(b[i]-j)/p[i] });}
}
for(int j=b[i]+p[i]; j<=n; j+=p[i]){
if(dog[j]){
//g[j].pb({b[i], abs(b[i]-j)/p[i] });
g[b[i] ].pb({j, abs(b[i]-j)/p[i] });}
}
crd[i ]=b[i];
}
d[crd[1] ]=0;
dijkstra();
cout<<d[crd[2] ];
return 0;
}
