#include <iostream>
#include<string>
#include<cmath>
#include<map>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<set>
#include<cstdio>
#include<stack>
#include<ctime>
#include<queue>
#include<deque>
#include<bitset>
#include<random>
#include<fstream>
#include<unordered_map>
#include<unordered_set>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using uint = unsigned int;
using dbl = double;
# define all(x) x.begin(), x.end()
# define rall(x) x.rbegin(), x.rend()
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
ll gcd(ll a, ll b) {
return b == 0 ? a : gcd(b, a % b);
}
ll lcm(ll a, ll b) {
return a / gcd(a, b) * b;
}
ll bpow(ll a, ll n) {
return n == 0 ? 1 : n % 2 ? bpow(a, n - 1) * a : bpow(a * a, n / 2);
}
ll bpowm(ll a, ll n, ll m) {
return n == 0 ? 1 : n % 2 ? bpowm(a, n - 1, m) * a % m : bpowm(a * a % m, n / 2, m);
}
mt19937 mt(time(0));
const int N = 3e4 + 2, SZ = 8e6 + 2, M = 5e6 + 2, inf = 1e9 + 100, mod = 1e9 + 7, mod2 = 998244353, P = 257;
const ll llinf = 1e18 + 1000;
int n, m, pt, dist[SZ];
vector<pair<int, int>> g[SZ];
map<int, vector<int>> a[N];
vector<int> vert[M];
void solve() {
cin >> n >> m;
pt = n;
int st, ansx, ansb;
for (int i = 0; i < m; ++i) {
if (i == 1) {
cin >> ansx >> ansb;
} else {
int x, b;
cin >> x >> b;
if (i == 0) st = x;
a[b][x % b].push_back(x);
}
}
for (int b = 1; b < n; ++b) {
for (auto x : a[b]) {
int p = x.first;
vector<int> pos = x.second;
int j = 0;
for (int i = p; i < n; i += b) {
if (j + 1 < pos.size() && abs(i - pos[j]) > abs(i - pos[j + 1])) ++j;
int d = abs(i - pos[j]) / b;
if(d == 0) g[i].push_back({ pt, 0 });
g[pt].push_back({ i, 0 });
if (i > p) {
g[pt - 1].push_back({ pt, 1 });
g[pt].push_back({ pt - 1, 1 });
}
++pt;
}
}
}
fill(dist, dist + pt, inf);
dist[st] = 0;
vert[0].push_back(st);
for (int d = 0; d < M; ++d) {
while(!vert[d].empty()) {
int v = vert[d].back();
vert[d].pop_back();
if (dist[v] < d) continue;
for (auto p : g[v]) {
int to = p.first, w = p.second;
if (d + w < dist[to]) {
dist[to] = d + w;
vert[dist[to]].push_back(to);
}
}
}
}
cout << (dist[ansx] == inf ? -1 : dist[ansx]);
}
signed main() {
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
//freopen("coloring.in", "r", stdin);
//freopen("coloring.out", "w", stdout);
//ld time1 = clock();
int tt = 1;
//cin >> tt;
while (tt--) {
solve();
cout << '\n';
cout.flush();
}
//ld time2 = clock();
//cerr << "\n\nTIME: " << (time2 - time1) / CLOCKS_PER_SEC;
return 0;
}
