#include <bits/stdc++.h>
using namespace std;
const int N = 30005;
const int B = 205;
const int INF = 0x3f3f3f3f;
int n, m;
int b[N], p[N];
int f[N][B];
vector<int> vec[N];
struct State {
int w, x, y;
State (int w, int x, int y) : w(w), x(x), y(y) {}
bool operator < (const State& rhs) const {
return w > rhs.w;
}
};
int solve() {
memset(f, INF, sizeof f);
priority_queue<State> pq;
pq.push(State(0, b[0], 0));
f[b[0]][0] = 0;
while (pq.size()) {
State u = pq.top(); pq.pop();
if (u.w != f[u.x][u.y]) continue;
if (u.x == b[1]) return u.w;
if (u.y) {
if (f[u.x][0] > u.w) {
f[u.x][0] = u.w, pq.push(State(u.w, u.x, 0));
}
if (u.x + u.y < n && f[u.x + u.y][u.y] > u.w + 1) {
f[u.x + u.y][u.y] = u.w + 1, pq.push(State(u.w + 1, u.x + u.y, u.y));
}
if (u.x - u.y >= 0 && f[u.x - u.y][u.y] > u.w + 1) {
f[u.x - u.y][u.y] = u.w + 1, pq.push(State(u.w + 1, u.x - u.y, u.y));
}
}
else {
for (auto i : vec[u.x]) {
if (p[i] >= B) {
for (int j = u.x % p[i]; j < n; j += p[i]) {
if (f[j][0] > u.w + abs(u.x - j) / p[i]) {
f[j][0] = u.w + abs(u.x - j) / p[i], pq.push(State(f[j][0], j, 0));
}
}
}
else {
if (f[u.x][p[i]] > u.w) {
f[u.x][p[i]] = u.w, pq.push(State(u.w, u.x, p[i]));
}
}
}
}
}
return -1;
}
int main() {
ios::sync_with_stdio(false);
cin >> n >> m;
for (int i = 0; i < m; ++i) {
cin >> b[i] >> p[i], vec[b[i]].push_back(i);
}
cout << solve();
}
