#include "bits/stdc++.h"
using namespace std;
const int MAXN = 6e6 + 10;
const int MOD = 1e9 + 7;
#define ll __int128
mt19937_64 rng((int)std::chrono::steady_clock::now().time_since_epoch().count());
int rnd(int x, int y) {
int u = uniform_int_distribution<int>(x, y)(rng); return u;
}
ll read() { int x; cin >> x; return (ll)x; }
long long bm(long long b, long long p) {
if(p==0) return 1 % MOD;
long long r = bm(b, p >> 1);
if(p&1) return (((r*r) % MOD) * b) % MOD;
return (r*r) % MOD;
}
long long inv(long long b) {
return bm(b, MOD-2);
}
int fastlog(int x) {
return (x == 0 ? -1 : 64 - __builtin_clzll(x) - 1);
}
void gay(int i) { cout << "Case #" << i << ": "; }
int csb(int l, int r, int k) { // count number of [l, r] such that i & 2^k > 0
if(l > r) return 0;
if(l == 0) {
int s = r / (1ll << (k+1)); // number of complete cycles
int t = r % (1ll << (k+1));
int ans = s * (1ll << k);
ans += (t >= (1ll << k) ? t - (1ll << k) + 1 : 0);
return ans;
}
else return csb(0, r, k) - csb(0, l - 1, k);
}
int lis(vector<int> a) {
int n = a.size();
int bucket[n+1];
for(int i=1; i<=n; i++) bucket[i] = 1e18;
int ans = 1;
for(int x: a) {
auto it = lower_bound(bucket + 1, bucket +n +1, x);
int d = distance(bucket, it);
ans = max(ans, d);
bucket[d] = min(bucket[d], x);
}
return ans;
}
vector<int> adj[MAXN];
void solve(int tc) {
int n, m;
cin >> n >> m;
//n = m = 30000;
int nxt = n;
int sq = ceil(sqrt(n));
int link[n][sq];
for(int i=1; i<sq; i++) {
for(int j=0; j<i; j++) {
for(int k=j; k<n; k+=i) {
link[k][i] = nxt;
nxt++;
}
for(int k=j; k<n; k+=i) {
if(k+i < n) {
adj[link[k][i]].push_back(link[k+i][i]);
adj[link[k+i][i]].push_back(link[k][i]);
}
adj[link[k][i]].push_back(k);
}
}
}
int st, en;
for(int i=0; i<m; i++) {
//cout << "i = " << i << "\n";
int b, p;
//b = rnd(0, n - 1), p = rnd(1, n);
cin >> b >> p;
if(i == 0) st = b;
else if(i == 1) en = b;
if(p >= sq) {
for(int j=b%p; j<n; j+=p) {
link[j][0] = nxt;
nxt++;
}
for(int j=b%p; j<n; j+=p) {
if(j+p < n) {
adj[link[j][0]].push_back(link[j+p][0]);
adj[link[j+p][0]].push_back(link[j][0]);
}
adj[link[j][0]].push_back(j);
}
adj[b].push_back(link[b][0]);
}
else {
adj[b].push_back(link[b][p]);
}
}
priority_queue<pair<int, int>,vector<pair<int, int> >,greater<pair<int, int> > > pq;
int dist[nxt];
bool vis[nxt];
for(int i=0; i<nxt; i++) vis[i] = 0, dist[i] = 1e9;
dist[st] = 0;
pq.push({0, st});
while(pq.size()) {
pair<int, int> t = pq.top(); pq.pop();
if(!vis[t.second]) {
vis[t.second] = 1;
for(int x: adj[t.second]) {
int w = (min(x, t.second) >= n);
if(!vis[x] && dist[x] > dist[t.second] + w) {
dist[x] = dist[t.second] + w;
pq.push({dist[x], x});
}
}
}
}
cout << (dist[en] == 1e9 ? -1 : dist[en]) << "\n";
}
int32_t main() {
ios::sync_with_stdio(0); cin.tie(0);
int t = 1; //cin >> t;
for(int i=1; i<=t; i++) solve(i);
}
// I don't know geometry.
// Teaming is unfair.
