#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<endl
#define dout(a) cout<<a<<' '<<#a<<endl
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<endl
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
const int mxn = 30005;
vi f(int x){
vi r;
FOR(i, 1, floor(sqrt(x) + 1)){
if(x % i == 0)r.pb(i);
}
int s = r.size();
f0r(i, s){
if(r[i] * r[i] != x){
r.pb(x / r[i]);
}
}
sort(rall(r));
return r;
}
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
vvi facts(mxn);
FOR(i, 1, mxn){
facts[i] = f(i);
}
//ifstream cin(".in");
//ofstream cout(".out");
in2(n,m);
vi b(m), p(m);
vector<set<int>>d(n);
f0r(i, m){
cin>>b[i]>>p[i]; d[b[i]].insert(p[i]);
}
vector<vpii> adj(n);
f0r(i, n){
f0r(j, n){
if(i != j){
for(auto fac : facts[abs(i - j)]){
if(d[i].count(fac)){
adj[i].pb(mp(j, abs(i - j) / fac));
break;
}
}
}
}
}
priority_queue<pii> q;
q.push(mp(0, b[0]));
vi dist(n,4e18); dist[b[0]] = 0;
while(!q.empty()){
int node = q.top().second; q.pop();
for(auto u : adj[node]){
int b = u.first; int w = u.second;
if(dist[b] > dist[node] + w){
dist[b] = dist[node] + w;
q.push(mp(-dist[b], b));
}
}
}
if(dist[b[1]] == 4e18)out(-1);
else out(dist[b[1]]);
}
