#include <bits/stdc++.h>
#define fr first
#define sc second
#define pii pair<int, int>
#define pb push_back
#define szof(s) (int)s.size()
#define all(s) s.begin(), s.end()
#define fastInp ios_base::sync_with_stdio(0); cin.tie(0);
#define int long long
using namespace std;
const int MAXN = (int)2e5 + 5;
const int INF = 1e18;
int pos[MAXN], p[MAXN];
vector <pii> g[MAXN];
int n, m;
void create(int pos, int x) {
if (x == 0) { // wtf
return;
}
for (int i = pos % x; i < n; i += x) {
if (i != pos) {
g[pos].pb({abs(pos - i) / x, i});
}
}
}
int dist[MAXN];
signed main() {
fastInp;
cin >> n >> m;
for (int i = 0; i < m; i++) {
cin >> pos[i] >> p[i];
create(pos[i], p[i]);
}
memset(dist, -1, sizeof(dist));
dist[pos[0]] = 0;
queue <pii> pq;
pq.push({0, pos[0]});
while (!pq.empty()) {
int v = pq.front().sc;
int cost = -pq.front().fr;
pq.pop();
if (cost > dist[v]) {
continue;
}
for (auto el : g[v]) {
int c = el.fr, to = el.sc;
if (dist[to] == -1 || dist[to] > dist[v] + c) {
dist[to] = dist[v] + c;
pq.push({-dist[to], to});
}
}
}
cout << dist[pos[1]] << endl;
}
/*
4 4
1 1
0 1
3 4
2 0
*/
