#include <bits/stdc++.h>
#define int long long
#define double long double
#define x first
#define y second
#define pb push_back
#define rt return
using namespace std;
signed main(){
int n,m;
cin>>n>>m;
vector <pair <int,int> > g[n];
pair <int,int> a[m];
for (int i=0; i<m; i++){
cin>>a[i].x>>a[i].y;
for (int j=1; a[i].x-a[i].y*j>=0; j++){
g[a[i].x].pb({a[i].x-a[i].y*j,j});
}
for (int j=1; a[i].x+a[i].y*j<n; j++){
g[a[i].x].pb({a[i].x+a[i].y*j,j});
}
}
priority_queue <pair <int,int>,vector <pair <int,int> >,greater <pair <int,int> > > pq;
bool visited[n];
for (int i=0; i<n; i++) visited[i]=0;
int dist[n];
for (int i=0; i<n; i++) dist[i]=1e18;
dist[a[0].x]=0;
pq.push({a[0].x,0});
while (!pq.empty()){
pair <int,int> tp=pq.top();
pq.pop();
if (visited[tp.y]) continue;
visited[tp.y]=1;
for (auto&i:g[tp.y]){
if (!visited[i.x]&&tp.x+i.y<dist[i.x]){
dist[i.x]=tp.x+i.y;
pq.push({dist[i.x],i.x});
}
}
}
if (dist[a[1].x]==1e18) cout<<"-1\n";
else cout<<dist[a[1].x]<<endl;
}
