#include <bits/stdc++.h>
using namespace std;
int n, m, dist[30002], poz;
set<int>s[30002];
bool viz[30002];
int main()
{
cin >> n >> m;
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int> > > q;
for (int i = 0; i < m; i++) {
int b, p;
cin >> b >> p;
if (i == 1)
poz = b;
s[b].insert(p);
if (i == 0)
q.push({0, b});
}
for (int i = 0; i < n; i++)
dist[i] = 1e9;
dist[q.top().second] = 0;
while (!q.empty()) {
auto x = q.top();
q.pop();
if (dist[x.second] != x.first)
continue;
if (x.second == poz)
break;
for (auto it : s[x.second]) {
for (int i = x.second+it; i < n; i += it) {
if (s[i].size() && dist[i] > dist[x.second] + (i - x.second) / it) {
dist[i] = dist[x.second] + (i - x.second) / it;
q.push({dist[i], i});
}
if (s[i].count(it))
break;
}
for (int i = x.second-it; i >= 0; i -= it) {
if (s[i].size() && dist[i] > dist[x.second] + (x.second - i) / it) {
dist[i] = dist[x.second] + (x.second - i) / it;
q.push({dist[i], i});
}
if (s[i].count(it))
break;
}
}
}
if (dist[poz] == 1e9)
dist[poz] = -1;
cout << dist[poz];
return 0;
}
