#include <bits/stdc++.h>
using namespace std;
#define bit(i, x) (x >> i & 1)
#define ll long long
#define sz(x) (int)x.size()
const int N = 30005;
const int mod = 998244353;
const int S = 173;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll rnd(ll l, ll r) {
return uniform_int_distribution<ll>(l, r)(rng);
}
int n, m;
pair<int, int> a[N];
int d[N][S + 5];
vector<int> adj[N];
pair<int, int> s, t;
priority_queue<array<int, 3>, vector<array<int, 3>>, greater<array<int, 3>>> q;
int res, INF;
bool vst[N];
bool minimize(int &x, int y) {
if(x <= y) return 0;
x = y;
return 1;
}
signed main(int argc, char* argv[]) {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
memset(d, 0x3f, sizeof d);
INF = d[0][0];
cin >> n >> m;
for(int i = 0; i < m; i++) {
cin >> a[i].first >> a[i].second;
adj[a[i].first].push_back(a[i].second);
if(i == 0) s = {a[i].first, 0};
else if(i == 1) t = {a[i].first, 0};
}
d[s.first][0] = 0;
q.push({0, s.first, 0});
while(!q.empty()) {
auto [val, u, k] = q.top();
q.pop();
assert(k <= S);
if(val > d[u][k]) continue;
// cout << u << " " << k << "\n";
if(k == 0) {
for(int k2 : adj[u]) {
if(k2 <= S) {
if(minimize(d[u][k2], val)) q.push({val, u, k2});
}
else {
for(int v = u % k2; v <= n - 1; v += k2) {
int dist = abs(u - v);
assert(dist % k2 == 0);
dist /= k2;
if(minimize(d[v][0], val + dist)) q.push({d[v][0], v, 0});
}
}
}
continue;
}
if(k <= S) {
if(u - k >= 0) {
if(minimize(d[u - k][0], val + 1)) q.push({d[u - k][0], u - k, 0});
if(minimize(d[u - k][k], val + 1)) q.push({d[u - k][k], u - k, k});
}
if(u + k <= n - 1) {
if(minimize(d[u + k][0], val + 1)) q.push({d[u + k][0], u + k, 0});
if(minimize(d[u + k][k], val + 1)) q.push({d[u + k][k], u + k, k});
}
}
else {
for(int v = u % k; v <= n - 1; v += k) {
int dist = abs(u - v);
assert(dist % k == 0);
dist /= k;
if(minimize(d[v][0], val + dist)) q.push({d[v][0], v, 0});
}
}
}
res = d[t.first][0];
if(res == INF) res = -1;
cout << res;
// cout << s.first << " " << s.second;
// cout << "\n";
// cout << t.first << " " << t.second;
return 0 ^ 0;
}
