#include <bits/stdc++.h>
using namespace std;
#define form2(i, a, b) for (int i = (a); i < (b); ++i)
#define ford2(i, a, b) for (int i = (a-1); i >= (b); --i)
#define form(i, n) form2(i, 0, n)
#define ford(i, n) ford2(i, n, 0)
#define chmax(x, v) x = max(x, (v))
#define chmin(x, v) x = min(x, (v))
#define fi first
#define se second
const int BIG = (int)(1e9);
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
void solve();
signed main()
{
ios::sync_with_stdio(false);
cin.tie(0);
solve();
return 0;
}
const int borneSuj = 30*1000 + 5;
const int borneTab = 200*borneSuj;
int gt[borneSuj][200];
int inv[borneTab];
bool witPrev[borneTab];
bool witSuiv[borneTab];
int arr = -1, dep=-1;
int nbPlaces, nbDoges, nbNoeuds;
int limSqd;
vector<pii> adj[borneSuj];
void solve()
{
cin >> nbPlaces >> nbDoges;
chmax(nbPlaces, 20);
nbNoeuds = nbPlaces;
limSqd = sqrt(nbPlaces);
form2(distance, 1, limSqd) {
form(ini, distance) {
for (int place = ini; place < nbPlaces; place += distance) {
int corNod = nbNoeuds; ++nbNoeuds;
inv[corNod] = place+1;
gt[place][distance] = corNod;
if (place != ini) {
witPrev[corNod] = true;
witSuiv[corNod-1] = true;
}
}
}
}
form(iDoge, nbDoges) {
int pos, saut;
cin >> pos >> saut;
if (iDoge == 0) dep = pos;
if (iDoge == 1) arr = pos;
if (saut < limSqd) {
adj[pos].push_back({gt[pos][saut], 0});
} else {
for (int dd = 1; pos + dd*saut < nbPlaces; ++dd) {
adj[pos].push_back({pos+dd*saut, dd});
}
for (int dd = 1; pos - dd*saut >= 0; ++dd) {
adj[pos].push_back({pos - dd*saut, dd});
}
}
}
vector<int> md(nbNoeuds, BIG);
md[dep] = 0;
priority_queue<pii, vector<pii>, greater<pii>> pq;
pq.push({0,dep});
while (! pq.empty()) {
int dis = pq.top().fi, nod = pq.top().se;
pq.pop();
if (dis != md[nod]) continue;
auto propag = [&] (int vo, int poids) {
if (dis + poids < md[vo]) {
md[vo] = dis+poids;
pq.push({md[vo], vo});
}
};
if (nod < nbPlaces) {
for (auto voRaw : adj[nod]) {
propag(voRaw.fi, voRaw.se);
}
}
if (nod >= nbPlaces) {
if (inv[nod]) propag(inv[nod]-1, 0);
if (witPrev[nod]) propag(nod-1, 1);
if (witSuiv[nod]) propag(nod+1, 1);
}
}
if (md[arr] == BIG) cout << "-1\n";
else cout << md[arr] << "\n";
}
