#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<vector<int>> p(n);
int f, s, z;
for (int i = 0; i < m; i++) {
int b, c;
cin >> b >> c;
if (i == 0) f = b, s = c;
if (i == 1) z = b;
if (c < n) p[b].push_back(c);
}
for (int i = 0; i < n; i++) {
sort(p[i].begin(), p[i].end());
p[i].erase(unique(p[i].begin(), p[i].end()), p[i].end());
}
const int inf = 1e9;
const int B = 180;
vector<vector<pair<int, int>>> adj(n);
for (int i = 0; i < n; i++) {
for (int j : p[i]) {
if (j >= B) {
int pos = i + j;
while (pos < n) {
adj[i].emplace_back(pos, (pos - i) / j);
pos += j;
}
pos = i - j;
while (pos >= 0) {
adj[i].emplace_back(pos, (i - pos) / j);
pos -= j;
}
}
}
while (!p[i].empty() && p[i].back() >= B) p[i].pop_back();
}
vector<vector<int>> d(n, vector<int>(B, inf));
d[f][0] = 0;
priority_queue<tuple<int, int, int>> q;
q.emplace(0, f, 0);
while (!q.empty()) {
auto [dis, u, v] = q.top();
if (u == z) {
cout << d[u][v];
return 0;
}
q.pop();
dis = -dis;
if (dis != d[u][v]) continue;
if (!v) {
for (int i : p[u]) {
if (d[u][i] > d[u][v]) {
d[u][i] = d[u][v];
q.emplace(-d[u][i], u, i);
}
}
}
else {
if (d[u][v] < d[u][0]) {
d[u][0] = d[u][v];
q.emplace(-d[u][0], u, 0);
}
if (u + v < n && d[u + v][v] > d[u][v] + 1) {
d[u + v][v] = d[u][v] + 1;
q.emplace(-d[u + v][v], u + v, v);
}
if (u - v >= 0 && d[u - v][v] > d[u][v] + 1) {
d[u - v][v] = d[u][v] + 1;
q.emplace(-d[u - v][v], u - v, v);
}
}
for (auto [c, w] : adj[u]) {
if (d[c][0] < d[u][v] + w) {
d[c][0] = d[u][v] + w;
q.emplace(-d[c][0], c, 0);
}
}
}
cout << -1;
return 0;
}
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