#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
typedef vector<char> vc;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;
#define sep ' '
#define F first
#define S second
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define kill(x) {cout << x << endl;return;}
#define sz(x) int(x.size())
#define SP(x) setprecision(x)
#define mod(x) (1ll*x%M+M)%M
#define pq priority_queue
#define mid (l+r)/2
// #define mid2 (l+r+1)/2
#define pll pair<ll, ll>
#define REP(i, k) for (int i = 0 ; i < k ; i ++)
#define MP make_pair
const int M = 998244353;
const int N = 1e5+1;
int n, m;
vector<pair<int, int>> adj[N];
ll dis[N];
bool vis[N];
void solve() {
// input in 0-index itself
cin >> n >> m;
for(int i = 0 ; i < m ; i ++) {
int b, p; cin >> b >> p;
for(int j = b+p, cnt = 1 ; j < n ; j += p, cnt++) {
adj[b].pb({j, cnt});
}
for(int j = b-p, cnt = 1 ; j >= 0 ; j -= p, cnt ++) {
adj[b].pb({j, cnt});
}
}
fill(dis, dis+n, 5e18);
dis[0] = 0;
for(int _ = 0 ; _ < n ; _ ++) {
ll mn = 5e18;
int v = -1;
for(int i = 0 ; i < n ; i ++) {
if (!vis[i] && dis[i] < mn) {
mn = dis[i];
v = i;
}
}
if (v == -1) break;
vis[v] = true;
for(auto [neigh, w]: adj[v])
if (!vis[neigh]) dis[neigh] = min(dis[neigh], dis[v] + w);
}
cout << (dis[1] == 5e18 ? -1 : dis[1]) << endl;
}
// check MAXN
int32_t main() {
fastIO;
solve();
return 0;
}
