#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 = 1e9 + 18;
const int maxn = 2e3 + 7, maxm = 3e4 + 7, mod = 1e9 + 7; //998244353
int n, m;
int b[maxm], p[maxm], val[maxn];
vector<pii> 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});
}
}
}
}
int32_t main(){
FAST_IO
cin >> n >> m ;
for(int i = 1 ; i <= m ; i ++){
cin >> b[i] >> p[i] ; b[i] ++;
}
for(int i = 1 ; i <= m ; i ++){
for(int j = 1 ; j <= n ; j ++){
if(abs(b[i] - j) % p[i] == 0){
adj[b[i]].pb({abs(b[i] - j)/p[i], j});
}
}
}
dij(b[1]);
if(val[b[2]] == inf) cout << -1 << endl;
else cout << val[b[2]] << endl;
return 0;
}
