#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<int> X(m), P(n);
for (int i = 0; i < m; i++)
cin >> X[i] >> P[X[i]];
vector<int> dis(n, INT_MAX); dis[X[0]] = 0;
priority_queue<pair<int, int>> q; q.push({0, X[0]});
while (!q.empty()) {
auto [d, x] = q.top(); q.pop();
if (x == X[1]) return cout << dis[x], 0;
if (-d != dis[x]) continue;
for (int i = x; i > 0; i -= P[x]) {
if (P[i] && dis[x] + (x - i) / P[x] < dis[i]) {
dis[i] = dis[x] + (x - i) / P[x];
q.push({-dis[i], i});
}
}
for (int i = x; i < n; i += P[x]) {
if (P[i] && dis[x] + (i - x) / P[x] < dis[i]) {
dis[i] = dis[x] + (i - x) / P[x];
q.push({-dis[i], i});
}
}
}
cout << -1;
}
