#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast,O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,sse4,sse4.2,lzcnt,popcnt")
#define X first
#define Y second
#define SZ(x) int(x.size())
#define pb push_back
#define Mp make_pair
using ll = long long;
using pii = pair<int, int>;
const int INF = 1e9;
int N;
vector<vector<pair<int, bool>>> g;
vector<pair<pii, bool>> E;
void add_edge(int u, int v, bool w) {
E.pb(Mp(pii(u,v), w));
}
vector<int> BFS01(int s) {
g.resize(N);
for(auto e : E)
g[e.X.X].pb(Mp(e.X.Y, e.Y));
vector<int> dis(N, INF);
dis[s] = 0;
deque<int> pq;
pq.push_back(s);
while(SZ(pq)) {
int v = pq.front();
pq.pop_front();
for(auto [u, w] : g[v])
if(dis[u]>dis[v]+w) {
dis[u] = dis[v]+w;
if(w) pq.push_back(u);
else pq.push_front(u);
}
}
return dis;
}
const int MXN = 3e4+4;
int n, m, B[MXN], P[MXN];
set<pii> st[MXN], chk;
inline int id(int i, int p) {
auto it = st[i].lower_bound(pii(p, 0));
if(it != st[i].end() && it->X==p) return it->Y;
int v = N++;
st[i].insert(pii(p, v));
return v;
}
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
cin >> n >> m;
N=n;
for(int i=0; i<m; i++) {
cin >> B[i] >> P[i];
int b = B[i]%P[i];
if(chk.find(pii(b,P[i]))==chk.end()) {
for(int j=b; j<n; j+=P[i]) {
add_edge(id(j, P[i]), j, 0);
if(j+P[i]<n)
add_edge(id(j, P[i]), id(j+P[i], P[i]), 1),
add_edge(id(j+P[i], P[i]), id(j, P[i]), 1);
}
chk.insert(pii(b,P[i]));
}
add_edge(B[i], id(B[i], P[i]), 0);
}
int ans = BFS01(id(B[0], P[0]))[id(B[1],P[1])];
if(ans == INF) ans = -1;
cout << ans << '\n';
return 0;
}
