#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define fi first
#define se second
#define ll long long
#define ld long double
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define mpp make_pair
#define ve vector
using namespace std;
using namespace __gnu_pbds;
template<class T> using oset = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
const ll inf = 1e18; const int iinf = 1e9;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
template <typename T> inline bool chmin(T& a, T b) { return (a > b ? a = b, 1 : 0); }
template <typename T> inline bool chmax(T& a, T b) { return (a < b ? a = b, 1 : 0); }
inline void solve() {
int n, m;
cin >> n >> m;
int fir = -1, sec = -1;
ve<ve<int>> lst(n);
for (int i = 0; i < m; ++i) {
int b, p;
cin >> b >> p;
lst[b].pb(p);
if (!i) fir = b;
else if (i == 1) sec = b;
}
ve<int> d(n, iinf);
d[fir] = 0;
set<pii> st;
st.insert({0, fir});
while (st.size()) {
int v = st.begin()->se;
st.erase(st.begin());
for (auto &p : lst[v]) {
int pos, cur;
pos = v + p, cur = 1;
while (pos < n) {
if (d[pos] > d[v] + cur) {
st.erase({d[pos], pos});
d[pos] = d[v] + cur;
st.insert({d[pos], pos});
}
pos += p;
++cur;
}
pos = v - p, cur = 1;
while (pos >= 0) {
if (d[pos] > d[v] + cur) {
st.erase({d[pos], pos});
d[pos] = d[v] + cur;
st.insert({d[pos], pos});
}
pos -= p;
++cur;
}
}
}
cout << (d[sec] == iinf ? -1 : d[sec]);
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int q = 1; // cin >> q;
while (q--) solve();
cerr << fixed << setprecision(3) << "Time execution: " << (double)clock() / CLOCKS_PER_SEC << endl;
}
