// In the name of the God
#include <bits/stdc++.h>
#define ll long long
// #define int long long
#define pb push_back
#define F first
#define S second
#define mp make_pair
#define pii pair <int, int>
#define smin(x, y) (x) = min((x), (y))
#define smax(x, y) (x) = max((x), (y))
#define all(x) (x).begin(), (x).end()
using namespace std;
const int inf = 1e9+7;
const int mod = 998244353;
const int maxn = 3e4+5;
int n, m, s, t, cnt = 0, dist[maxn];
set <int> doge[maxn];
set <pii> st;
void dij(int v) {
fill(dist, dist+n, inf);
dist[v] = 0;
st.insert(mp(0, v));
while (st.size()) {
int d = st.begin()->F, u = st.begin()->S;
st.erase(st.begin());
for (int p : doge[u]) {
for (int j = 1; u+j*p < n; j++) {
if (dist[u+j*p] > d+j) {
st.erase(mp(dist[u+j*p], u+j*p));
dist[u+j*p] = d+j;
st.insert(mp(d+j, u+j*p));
}
cnt++; assert(cnt < 7381230);
if (doge[u+j*p].find(p) != doge[u+j*p].end()) break;
}
for (int j = 1; u-j*p >= 0; j++) {
if (dist[u-j*p] > d+j) {
st.erase(mp(dist[u-j*p], u-j*p));
dist[u-j*p] = d+j;
st.insert(mp(d+j, u-j*p));
}
cnt++; assert(cnt < 7381230);
if (doge[u-j*p].find(p) != doge[u-j*p].end()) break;
}
}
}
}
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n >> m;
for (int i = 0; i < m; i++) {
int a, b; cin >> a >> b;
doge[a].insert(b);
if (i == 0) s = a;
if (i == 1) t = a;
}
dij(s);
if (dist[t] != inf) assert(dist[t] <= 1000000);
cout << (dist[t] == inf ? -1 : dist[t]) << '\n';
// int sum = 0, i;
// for ( i = 1; sum <= maxn; i++) sum += i;
// cout << i << '\n';
// sum = 0;
// for (int j = 1; j <= i; j++) sum += maxn;
// cout << sum << '\n';
return 0;
}
