#include<bits/stdc++.h>
using namespace std;
const int MAXN = 3e4 + 5;
const int block = 170;
#define pii pair<int,int>
#define int long long
int n, m;
vector<int> adj[MAXN];
int b[MAXN];
int p[MAXN];
int start = 0;
int endd = 0;
int d[MAXN];
bool ck[MAXN][block];
void dijskstra()
{
for (int i = 0; i < n; i++)
{
d[i] = INT_MAX;
}
d[start] = 0;
priority_queue<pii, vector<pii>, greater<pii>> pq;
pq.push({d[start], start});
while(!pq.empty())
{
auto[dist, u] = pq.top();
pq.pop();
if (dist > d[u])
{
continue;
}
if (u == endd)
{
break;
}
for (int v : adj[u])
{
if (v >= block || (v < block and ck[u][v] == false))
{
if (v < block)
{
ck[u][v] = true;
}
for (int i = u + v; i < n; i += v)
{
if (d[i] > d[u] + (i - u) / v)
{
d[i] = d[u] + (i - u) / v;
pq.push({d[i], i});
}
}
for (int i = u - v; i >= 0; i -= v)
{
if (d[i] > d[u] + (u - i) / v)
{
d[i] = d[u] + (u - i) / v;
pq.push({d[i], i});
}
}
}
}
}
if (d[endd] == INT_MAX)
{
cout << -1;
return;
}
else
{
cout << d[endd];
}
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> m;
for (int i = 0; i < m; i++)
{
cin >> b[i] >> p[i];
adj[b[i]].push_back(p[i]);
}
start = b[0];
endd = b[1];
for (int i = 0; i < n; i++)
{
sort(adj[i].begin(), adj[i].end());
adj[i].erase(unique(adj[i].begin(), adj[i].end()), adj[i].end());
}
dijskstra();
}
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