#include <bits/stdc++.h>
using namespace std;
using TNode = pair<int64_t, int>;
const int N = 50005;
int p[N];
vector<int> g[N];
vector<int> rg[N];
int64_t d[N];
int n, m;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> m;
for (int i = 0; i < m; i++) {
int id_b;
cin >> id_b >> p[i];
g[i].emplace_back(id_b);
rg[id_b].emplace_back(i);
}
memset(d, 0x3f, sizeof(d));
d[0] = 0;
priority_queue<TNode, vector<TNode>, greater<TNode>> pq;
pq.emplace(d[0], 0);
while (!pq.empty()) {
int64_t du; int u;
tie(du, u) = pq.top();
pq.pop();
if (d[u] != du) {
continue;
}
if (u == 1) {
cout << du;
return 0;
}
for (int id_b : g[u]) {
for (int nxt_b = id_b; nxt_b >= 0; nxt_b -= p[u]) {
for (int v : rg[nxt_b]) {
int cost = abs(nxt_b - id_b) / p[u];
if (d[v] > d[u] + cost) {
d[v] = d[u] + cost;
pq.emplace(d[v], v);
}
}
}
for (int nxt_b = id_b; nxt_b < n; nxt_b += p[u]) {
for (int v : rg[nxt_b]) {
int cost = abs(nxt_b - id_b) / p[u];
if (d[v] > d[u] + cost) {
d[v] = d[u] + cost;
pq.emplace(d[v], v);
}
}
}
}
}
cout << -1;
return 0;
}
