#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = 1e9+7;
const ll MOD = 998244353;
typedef pair<ll,ll> ii;
#define iii pair<ii,ll>
#define f(i,a,b) for(ll i = a;i < b;i++)
#define pb push_back
#define vll vector<ll>
#define F first
#define S second
#define all(x) (x).begin(), (x).end()
///I hope I will get uprating and don't make mistakes
///I will never stop programming
///sqrt(-1) Love C++
///Please don't hack me
///@TheofanisOrfanou Theo830
///Think different approaches (bs,dp,greedy,graphs,shortest paths,mst)
///Stay Calm
///Look for special cases
///Beware of overflow and array bounds
///Think the problem backwards
///Training
const ll val = 200;
ll dist[30005][val+5];
int main(void){
ios_base::sync_with_stdio(0);
cin.tie(0);
ll n,m;
cin>>n>>m;
ll b[m],p[m];
vector<ll>ex[n];
f(i,0,m){
cin>>b[i]>>p[i];
ex[b[i]].pb(p[i]);
}
ll best[n];
f(i,0,n){
best[i] = INF;
f(j,0,val+5){
dist[i][j] = INF;
}
}
best[b[0]] = 0;
priority_queue<ii,vector<ii>,greater<ii> >pq,q[val+5];
pq.push(ii(0,b[0]));
while(!pq.empty()){
ii f = pq.top();
pq.pop();
ll w = f.F,u = f.S;
if(best[u] < w){
continue;
}
for(auto x:ex[u]){
if(x >= val){
ll cur = u;
ll posa = 0;
while(cur < n){
if(best[cur] > w + posa){
best[cur] = w + posa;
pq.push(ii(best[cur],cur));
}
cur += x;
posa++;
}
cur = u;
posa = 0;
while(cur >= 0){
if(best[cur] > w + posa){
best[cur] = w + posa;
pq.push(ii(best[cur],cur));
}
cur -= x;
posa++;
}
}
else{
if(dist[u][x] > w){
dist[u][x] = w;
q[x].push(ii(w,u));
}
}
}
f(j,0,val){
while(!q[j].empty()){
ii f = q[j].top();
q[j].pop();
ll x = f.F,y = f.S;
if(dist[y][j] < x){
continue;
}
ll pos = y - j;
if(pos >= 0 && dist[pos][j] > x+1){
dist[pos][j] = x+1;
q[j].push(ii(dist[pos][j],pos));
if(best[pos] > dist[pos][j]){
best[pos] = dist[pos][j];
pq.push(ii(best[pos],pos));
}
}
pos = y + j;
if(pos < n && dist[pos][j] > x+1){
dist[pos][j] = x+1;
q[j].push(ii(dist[pos][j],pos));
if(best[pos] > dist[pos][j]){
best[pos] = dist[pos][j];
pq.push(ii(best[pos],pos));
}
}
}
}
}
ll ans = best[b[1]];
if(ans == INF){
ans = -1;
}
cout<<ans<<"\n";
}
