#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef vector<ll> vll;
typedef vector <pair<ll, ll>> vp;
typedef pair<ll, ll> pll;
typedef map <ll, ll> mll;
typedef set <ll> sll;
#define pb push_back
#define ff first
#define ss second
#define str to_string
#define all(x) (x).begin(), (x).end()
#define print(x) for (auto i : x) cout << i << ' '; cout << '\n';
#define FastIO ios_base::sync_with_stdio(false); cin.tie(NULL);
const ll maxn = 3e4+5;
const ll mod = 1e9+7;
const ll inf = 1e18;
ll pw(ll a, ll b, ll M = mod) {ll ans = 1; for (; b; a = ((a * a) % M), b >>= 1) if (b & 1) ans = (ans * a) % M; return ans;}
ll n, m, b[maxn], p[maxn], dist[maxn];
vll dgs[maxn];
int main() {
FastIO
cin >> n >> m;
for (ll i = 0; i < m; i++) {
cin >> b[i] >> p[i];
dgs[b[i]].pb(i);
}
fill(dist+1, dist+m, inf);
queue <ll> q;
q.push(0);
while (!q.empty()) {
ll v = q.front();
q.pop();
for (ll i = b[v], d = 0; i >= 0; i -= p[v], d++) {
for (ll u : dgs[i]) {
if (dist[u] > dist[v] + d) {
dist[u] = dist[v] + d;
q.push(u);
}
}
}
for (ll i = b[v]+p[v], d = 1; i < n; i += p[v], d++) {
for (ll u : dgs[i]) {
if (dist[u] > dist[v] + d) {
dist[u] = dist[v] + d;
q.push(u);
}
}
}
}
if (dist[1] < inf) cout << dist[1] << '\n';
else cout << -1 << '\n';
return 0;
}
