#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 30005;
const int C = 300;
bool used[C][C] = {};
vector<int> steps[MAX_N];
vector<pair<int, int>> G[MAX_N * 2];
int B[MAX_N], P[MAX_N];
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
int N, M;
cin >> N >> M;
for (int i = 0; i < M; i++) {
cin >> B[i] >> P[i];
if (P[i] >= C) {
for (int j = B[i] % P[i]; j < N; j += P[i]) {
G[B[i]].push_back({j, abs(B[i] - j) / P[i]});
}
} else {
steps[B[i]].push_back(P[i]);
}
}
vector<int> dist(2 * N, INT_MAX);
priority_queue<pair<int, int>> Q;
Q.push({0, B[0]});
dist[B[0]] = 0;
while (!Q.empty()) {
auto [d, u] = Q.top();
Q.pop();
if (d > dist[u]) {
continue;
}
if (u < N) {
for (int step : steps[u]) {
int rem = u % step;
if (!used[step][rem]) {
for (int j = rem; j < N; j += step) {
int w = abs(u - j) / step;
if (dist[u] + w < dist[j]) {
dist[j] = dist[u] + w;
Q.push({dist[j], j});
}
}
used[step][rem] = true;
}
}
}
for (auto [v, w] : G[u]) {
if (dist[v] > dist[u] + w) {
dist[v] = dist[u] + w;
Q.push({dist[v], v});
}
}
}
cout << (dist[B[1]] == INT_MAX ? -1 : dist[B[1]]) << "\n";
}
