#include <bits/stdc++.h>
#include <exception>
using namespace std;
using ll = long long;
const int INF = 1e9 + 12;
const int K = 120;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<pair<int, int>> v(m);
for(auto &z : v) cin >> z.first >> z.second;
priority_queue<pair<int, pair<int, int>>, vector<pair<int, pair<int, int>>>, greater<>> pq;
pq.push({0, v[0]});
set<pair<int, int>> visited;
int ans = INF;
vector<pair<int, int>> where[n];
for(auto z : v) {
where[z.first].push_back(z);
}
while(!pq.empty()) {
auto best = pq.top();
pq.pop();
auto dist = best.first;
auto current = best.second;
if(visited.count(current)) continue;
// cerr << dist << " " << current.first << " " << current.second << endl;
if(current == v[1]) {
ans = min(ans, dist);
break;
}
visited.insert(current);
// where[current.first].erase(current);
if(current.second < K) {
for(auto z : where[current.first]) {
pq.push({dist, z});
}
if(current.second != 0) {
vector<pair<int, int>> go;
go.push_back({current.first - current.second, current.second});
go.push_back({current.first + current.second, current.second});
auto check = [&n](pair<int, int> x) {
return x.first >= 0 && x.first < n;
};
for(auto z : go) {
if(check(z)) {
pq.push({dist + 1, z});
}
}
}
} else {
for(int i = current.first % current.second; i < n; i += current.second) {
int raz = abs(i - current.first);
raz /= current.second;
for(auto z : where[i]) {
pq.push({dist + raz, z});
}
}
}
}
cout << ((ans >= INF) ? -1 : ans) << '\n';
}
