/// In The Name Of God
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
#define pp pop_back
#define mp make_pair
#define sz(x) (int)x.size()
#define sqr(x) ((x) * 1ll * (x))
#define all(x) x.begin(), x.end()
#define Kazakhstan ios_base :: sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define nl '\n'
#define ioi exit(0);
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int N = (int)3e4 + 7;
const int K = 175;
const int inf = (int)1e9 + 7;
const int mod = (int)1e9 + 7;
const ll linf = (ll)1e18 + 7;
const int dx[] = {-1, 0, 1, 0, 1, -1, -1, 1};
const int dy[] = {0, 1, 0, -1, 1, -1, 1, -1};
using namespace std;
int n, m;
int sta, fn;
int b[N], p[N];
vector < pair <int, int> > g[N];
set <int> a[N];
ll d[N];
int main() {
#ifdef IOI2018
freopen ("in.txt", "r", stdin);
#endif
Kazakhstan
cin >> n >> m;
for (int i = 1; i <= m; i++) {
cin >> b[i] >> p[i];
if (i == 1) sta = b[i];
if (i == 2) fn = b[i];
a[b[i]].insert(p[i]);
}
for (int i = 0; i < n; i++) {
for (auto it : a[i]) {
int p = i + it, cnt = 0;
while (p < n) {
g[i].pb({p, ++cnt});
if (a[p].count(it)) break;
p += it;
}
p = i - it, cnt = 0;
while (p >= 0) {
g[i].pb({p, ++cnt});
if (a[p].count(it)) break;
p -= it;
}
}
}
set < pair <ll, int > > st;
memset(d, 0x3f, sizeof(d));
st.insert({0, sta});
d[sta] = 0;
while (sz(st)) {
pair <ll, int> v = *st.begin();
st.erase(st.begin());
for (auto to : g[v.s]) {
if (d[to.f] > v.f + to.s) {
st.erase({d[to.f], to.f});
d[to.f] = v.f + to.s;
st.insert({d[to.f], to.f});
}
}
}
if (d[fn] >= linf) cout << -1;
else cout << d[fn];
ioi
}
