#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,ssse3,sse3,sse4,popcnt,avx,mmx,abm,tune=native")
#include <bits/stdc++.h>
using namespace std;
const int MN = 30004, MS = 200;
int N, M, S, i, j, src, des, x, y, ent[MN];
struct pir{int f, s, p;}; vector<pir> e[MN]; bitset<MS> pw[MN];
pir pr(int f, int s, int p){pir z={f,s,p}; return z;}
struct pq{bool operator()(const pir&i,const pir&j){return(i.s>j.s);}};
priority_queue<pir,vector<pir>,pq> q;
int main(){
for(scanf("%d%d",&N,&M),S=sqrt(N+0.0);i<M;i++){
scanf("%d%d",&x,&y);
if(i == 0) src = x;
else if(i == 1) des = x;
if(y >= S){
for(j=1;x-j*y>=0;j++)
e[x].push_back(pr(x-j*y,j,-1));
for(j=1;x+j*y<N;j++)
e[x].push_back(pr(x+j*y,j,-1));
}
else e[x].push_back(pr(x,0,y));
}
pir st = {src, 0, -1}; q.push(st);
while(!q.empty()){
pir _ = q.top(); q.pop();
if(_.f == des){printf("%d\n",_.s); return 0;}
if(_.p == -1){
if(ent[_.f]) continue;
else ent[_.f] = 1;
for(auto v : e[_.f]){
if(v.p == -1 && !ent[v.f]) q.push(pr(v.f,v.s+_.s,v.p));
else if(v.p != -1 && !pw[v.f][v.p]) q.push(pr(v.f,v.s+_.s,v.p));
}
}
else{
if(pw[_.f][_.p]) continue;
else pw[_.f][_.p] = 1;
if(!ent[_.f]) q.push(pr(_.f,_.s,-1));
if(_.f+_.p < N && !pw[_.f+_.p][_.p]) q.push(pr(_.f+_.p,_.s+1,_.p));
if(_.f-_.p >=0 && !pw[_.f-_.p][_.p]) q.push(pr(_.f-_.p,_.s+1,_.p));
}
}
printf("-1\n");
return 0;
}
