#include <bits/stdc++.h>
using namespace std;
#define int long long
#define inf 1e18
const int MXN = 3e4 + 5;
int b[MXN], p[MXN], dist[MXN];
vector<int> adj[MXN];
map<int, int> mp[MXN];
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
for (int i = 0; i < n; i++) dist[i] = inf;
for (int i = 0; i < m; i++)
{
cin >> b[i] >> p[i];
adj[b[i]].push_back(p[i]);
}
priority_queue<array<int, 2>, vector<array<int, 2>>, greater<array<int, 2>>> pq;
dist[b[0]] = 0;
pq.push({0, b[0]});
while (!pq.empty())
{
array<int, 2> f = pq.top();
pq.pop();
if (f[0] > dist[f[1]]) continue;
for (int v : adj[f[1]])
{
if (mp[v].find(f[1] % v) != mp[v].end()) continue;
mp[v][f[1] % v];
for (int i = f[1] - v; i >= 0; i -= v)
{
if (dist[i] > dist[f[1]] + ((f[1] - i) / v))
{
dist[i] = dist[f[1]] + ((f[1] - i) / v);
pq.push({dist[i], i});
}
}
for (int i = f[1] + v; i < n; i += v)
{
if (dist[i] > dist[f[1]] + ((i - f[1]) / v))
{
dist[i] = dist[f[1]] + ((i - f[1]) / v);
pq.push({dist[i], i});
}
}
}
}
cout << (dist[b[1]] >= inf ? -1 : dist[b[1]]) << '\n';
}
