#include <bits/stdc++.h>
using namespace std;
constexpr int inf = 1e9;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<int> b(m), p(m);
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
}
vector<vector<array<int, 2>>> g(n);
for (int i = 0; i < m; i++) {
for (int j = 0; j < m; j++) {
if (i == j) continue;
int x = abs(b[i] - b[j]);
if (x % p[i] == 0) {
g[b[i]].push_back({b[j], x / p[i]});
}
}
}
vector<int> d(n, inf);
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> q;
q.push({b[0], 0});
while (!q.empty()) {
auto [dist, u] = q.top();
q.pop();
if (d[u] < dist) continue;
d[u] = dist;
for (auto [v, w] : g[u]) {
if (dist + w < d[v]) {
d[v] = dist + w;
q.push({d[v], v});
}
}
}
cout << (d[b[1]] == inf ? -1 : d[b[1]]) << '\n';
return 0;
}
