#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()
#define MOO(i, a, b) for(int i=a; i<b; i++)
#define M00(i, a) for(int i=0; i<a; i++)
#define MOOd(i,a,b) for(int i = (b)-1; i >= a; i--)
#define M00d(i,a) for(int i = (a)-1; i>=0; i--)
#define FAST ios::sync_with_stdio(0); cin.tie(0);
#define finish(x) return cout << x << '\n', 0;
#define dbg(x) cerr << ">>> " << #x << " = " << x << "\n";
#define _ << " _ " <<
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int,int> pi;
typedef pair<ld,ld> pd;
typedef complex<ld> cd;
template<int SZ> struct dijkstra {
const int inf = 1e8;
vector<pi> adj[SZ];
int d[SZ];
void addEdge(int u, int v, int l) {
adj[u].pb(mp(v, l));
}
int dist(int v) {
return d[v];
}
void build(int u) {
priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<pair<ll, int>>> pq;
M00(i, SZ) d[i] = inf;
pq.push(mp(0, u));
while(!pq.empty()) {
pi t = pq.top(); pq.pop();
if(d[t.s] != inf) continue;
d[t.s] = t.f;
for(auto v: adj[t.s]) {
if(d[v.f] > d[t.s] + v.s) if(d[v.f] == inf) {
pq.push(mp(d[t.s]+v.s, v.f));
}
}
}
}
};
const int MAX_N = 30030;
int n, m;
set<int> doges[MAX_N];
dijkstra<MAX_N> dj;
int main() { FAST
cin >> n >> m;
int st = -1, en = -1;
M00(i, m) {
int b, p; cin >> b >> p;
if(i == 0) st = b;
else if(i == 1) en = b;
doges[b].insert(p);
}
M00(i, n) {
for(int j: doges[i]) {
for(int x = i+j; x < n; x += j) {
dj.addEdge(i,x,(x-i)/j);
if(doges[x].count(j)) break;
}
for(int x = i-j; x >= 0; x -= j) {
dj.addEdge(i,x,(i-x)/j);
if(doges[x].count(j)) break;
}
}
}
dj.build(st);
int res = dj.dist(en);
if(res == 1e8) {
finish(-1);
}
cout << dj.dist(en) << "\n";
}
