#include <cstdio>
#include <iostream>
#include <algorithm>
#include <string>
#include <cstring>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cmath>
#include <iomanip>
#include <cassert>
#include <bitset>
using namespace std;
typedef pair<int, int> P;
typedef pair<int, P> P2;
#define rep(i, n) for (int i=0; i<(n); i++)
#define all(c) (c).begin(), (c).end()
#define uniq(c) c.erase(unique(all(c)), (c).end())
#define index(xs, x) (int)(lower_bound(all(xs), x) - xs.begin())
#define _1 first
#define _2 second
#define pb push_back
#define INF 1145141919
#define MOD 1000000007
const int B = 180;
int N, M;
vector<int> G[30000];
int D[30000][B+1];
signed main() {
ios::sync_with_stdio(false); cin.tie(0);
cin >> N >> M;
rep(i, N) rep(j, B+1) D[i][j] = INF;
int sx = 0, tx = 0;
rep(i, M) {
int b, p;
cin >> b >> p;
if (i == 0) sx = b;
else if (i == 1) tx = b;
G[b].pb(p);
}
rep(i, N) sort(all(G[i])), uniq(G[i]);
priority_queue<P2, vector<P2>, greater<P2> > q;
D[sx][0] = 0;
q.push(P2(0, P(sx, 0)));
while (!q.empty()) {
int x = q.top()._2._1,
p = q.top()._2._2,
r = q.top()._1; q.pop();
if (D[x][p] < r) continue;
if (p == 0) {
for (int np : G[x]) {
if (np <= B) {
if (D[x][np] > r) {
D[x][np] = r;
q.push(P2(r, P(x, np)));
}
}
else {
for (int t=x%np; t<N; t+=np) {
int cost = abs(x-t)/np;
if (D[t][0] > r+cost) {
D[t][0] = r+cost;
q.push(P2(r+cost, P(t, 0)));
}
}
}
}
}
else {
if (D[x][0] > r) {
D[x][0] = r;
q.push(P2(r, P(x, 0)));
}
if (x-p >= 0) {
if (D[x-p][p] > r+1) {
D[x-p][p] = r+1;
q.push(P2(r+1, P(x-p, p)));
}
}
if (x+p < N) {
if (D[x+p][p] > r+1) {
D[x+p][p] = r+1;
q.push(P2(r+1, P(x+p, p)));
}
}
}
}
int m = D[tx][0];
if (m == INF) m = -1;
cout << m << "\n";
return 0;
}
