#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#define N 30005
struct A{
int a,b,c;
};
int a[N],p[N];
vector<int> g[N];
bitset<N> vis;
unordered_map<int,int> mp[N];
queue<A> q;
int main(){
int n,m,i;
scanf("%d %d",&n,&m);
for(i=0;i<m;i++){
scanf("%d %d",&a[i],&p[i]);
g[a[i]].push_back(p[i]);
}
q.push({a[0],p[0],0});
mp[a[0]][p[0]]=0;
while(!q.empty()){
auto [x,po,w]=q.front();
q.pop();
if(x==a[1]){printf("%d",w);return 0;}
if(vis[x])continue;
vis[x]=true;
if(x>=po&&!mp[x-po].count(po))q.push({x-po,po,w+1}),mp[x-po][po]=w+1;
if(x+po<n&&!mp[x+po].count(po))q.push({x+po,po,w+1}),mp[x+po][po]=w+1;
for(auto po:g[x]){
if(x>=po&&!mp[x-po].count(po))q.push({x-po,po,w+1}),mp[x-po][po]=w+1;
if(x+po<n&&!mp[x+po].count(po))q.push({x+po,po,w+1}),mp[x+po][po]=w+1;
}
}
printf("-1");
return 0;
}
