#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
#define st first
#define nd second
const int N = 3e4 + 4;
const int kok = 177;
typedef long long ll;
typedef pair < int , int > pp;
vector < pp > Q[N*kok];
vector < int > V[N];
int H[N][kok],F[N],beg[N],jum[N],n,m,i,j,x,u,res,ss;
void f(int x, int u, int t){
if(u >= kok){
for(int i=0;i<kok;i++){
if(x + i*u < n && !H[x+i*u][0])
ss++, Q[t+i].pb(mp(x+i*u,0));
if(x - i*u >= 0 && !H[x-i*u][0])
ss++, Q[t+i].pb(mp(x-i*u,0));
}
}
else if(!H[x][u]) ss++, Q[t].pb(mp(x,u));
}
int main(){
cin >> n >> m;
for(i=0;i<m;i++){
scanf("%d%d",&beg[i],&jum[i]);
V[ beg[i] ].pb( jum[i] );
}
f(beg[0],jum[0],0);
for(res=0 ; ss ; res++)
for(i=0;i<Q[res].size();i++,ss--){
x = Q[res][i].st;
u = Q[res][i].nd;
if(x == beg[1]) { cout << res; return 0; }
if(H[x][u]) continue;
H[x][u] = 1;
if(u){ if(x+u<n && !H[x+u][u])ss++,Q[res+1].pb(mp(x+u,u));
if(x-u>=0 && !H[x-u][u])ss++,Q[res+1].pb(mp(x-u,u)); }
if(F[x]) continue;
F[x] = 1;
for(j=0;j<V[x].size();j++)
f(x,V[x][j],res);
}
cout << -1;
return 0;
}
