#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
int n, m; cin >> n >> m;
vector<vector<bool>> used(n, vector<bool>(n+1, false));
vector<vector<pii>> adj(n);
vector<vector<int>> have(n);
vector<int> b(m), p(m);
for (int i=0; i<m; i++) {
cin >> b[i] >> p[i];
have[b[i]].push_back(p[i]);
}
int s = b[0];
//int e = b[1];
for (int i=0; i<n; i++) {
for (auto j : have[i]) {
if (used[i][j]) continue;
used[i][j] = true;
for (int c=1; c*j<=n; c++) {
int jump = c*j;
if (i+jump<n) adj[i].push_back({i+jump, c});
if (i-jump >= 0) adj[i].push_back({i-jump, c});
}
}
}
vector<int> dis(n, 1e9);
dis[s]=0;
priority_queue<pii, vector<pii>, greater<pii>> pq;
pq.push({0, s});
while (!pq.empty()) {
auto [d, u] = pq.top();
pq.pop();
if (d!=dis[u]) continue;
for (auto [v, w] : adj[u]) {
if (dis[v]>dis[u]+w) {
dis[v] = dis[u]+w;
pq.push({dis[v], v});
}
}
}
cout << (dis[b[1]] == 1e9 ? -1 : dis[b[1]]) << "\n";
}
