#include <bits/stdc++.h>
using namespace std;
const int N = 30'000 + 10;
int n, m;
int b[N], p[N];
vector<int> power[N];
int f[N];
int32_t main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> m;
for (int i = 0; i < m; ++i) cin >> b[i] >> p[i];
for (int i = 0; i < m; ++i) power[b[i]].push_back(p[i]);
memset(f, 63, sizeof f);
using pii = pair<int, int>;
priority_queue<pii, vector<pii>, greater<>> q;
q.push({f[b[0]] = 0, b[0]});
while (q.size()) {
auto [d, u] = q.top(); q.pop();
if (d != f[u]) continue;
for (const auto& pw : power[u]) {
{
int w = d;
for (int v = u + pw; v < n; v += pw) if (f[v] > ++w) q.push({f[v] = w, v});
}
{
int w = d;
for (int v = u - pw; v >= 0; v -= pw) if (f[v] > ++w) q.push({f[v] = w, v});
}
}
}
cout << (f[b[1]] < f[n] ? f[b[1]] : -1) << "\n";
}
