#include <bits/stdc++.h>
using namespace std;
// #include <ext/pb_ds/tree_policy.hpp>
// #include <ext/pb_ds/assoc_container.hpp>
// using namespace __gnu_pbds; // find_by_order order_of_key
// template<class T> using oset = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long ll;
// #pragma GCC optimize ("O2,unroll-loops")
// #pragma GCC optimize ("Ofast")
// #pragma GCC target("avx2,bmi,bmi2,popcnt,lzcnt")
#define int ll
#define FAST_IO (ios_base::sync_with_stdio(false), cin.tie(NULL));
#define file_init freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout);
#define testc int ttt;cin>>ttt;while(ttt--)
#define alll(x) (x).begin(),(x).end()
#define pqi priority_queue<pii, vector<pii>, greater<pii>>
#define endl '\n'
#define pb push_back
#define ins insert
#define pii pair<int, int>
#define ff first
#define ss second
#define bg begin
#define rbg rbegin
#define sz size()
#define mset(x,y) memset(x,y,sizeof(x))
#define clr clear()
#define YES cout<<"YES"<<endl
#define NO cout<<"NO"<<endl
#define vci vector<int>
#define sti set<int>
#define nl cout<<'\n'
#define upb upper_bound
#define lrb lower_bound
//ll pw(ll a,ll b){ll res=1;while(b){if(b&1)res=res*a%mod;a=a*a%mod;b>>=1;}return res;}
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b);}
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }
const ll inf = 1e18 + 18;
const int maxn = 3e4 + 7, mod = 1e9 + 7; //998244353
int n, m;
int p[maxn], b[maxn], val[maxn];
vci adj[maxn];
// void dij(int a){
// fill(val, val + n + 1, inf);
// val[a] = 0;
// pqi pq; pq.push({0, a});
// while(pq.sz){
// auto[ww, u] = pq.top(); pq.pop();
// if(ww > val[u]) continue;
// for(auto[w, v] : adj[u]){
// if(val[u] + w < val[v]){
// val[v] = val[u] + w;
// pq.push({val[v], v});
// }
// }
// }
// }
void deltek(){
for(int i = 0 ; i < n ; i ++){
sort(alll(adj[i]));
adj[i].erase(unique(alll(adj[i])), adj[i].end());
}
}
void dij(int a){
pqi pq;
fill(val, val + n + 1, inf); val[a] = 0;
pq.push({0, a});
while(pq.sz){
auto [ww, x] = pq.top(); pq.pop();
if(ww > val[x]) continue;
for(int item : adj[x]){
int dst = 0;
for(int i = x + item ; i < n ; i += item){ dst ++;
if(val[x] + dst < val[i]){
val[i] = val[x] + dst;
pq.push({val[i], i});
}
}
dst = 0;
for(int i = x - item ; i >= 0 ; i -= item){ dst ++;
if(val[x] + dst < val[i]){
val[i] = val[x] + dst;
pq.push({val[i], i});
}
}
}
}
}
int32_t main() {
FAST_IO
cin >> n >> m;
for(int i = 0 ; i < m ; i ++){
cin >> b[i] >> p[i]; adj[b[i]].pb(p[i]);
} deltek();
dij(b[0]);
if(val[b[1]] >= inf) val[b[1]] = -1;
cout << val[b[1]] << endl;
return 0;
}
