#include <algorithm>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#ifdef DEBUG
#include <time.h>
#endif
#define all(a) (a).begin(), (a).end()
#define rev(a) (a).rbegin(), (a).rend()
#define F first
#define S second
int recur_depth = 0;
#ifdef DEBUG
#define dbg(x) \
{ \
++recur_depth; \
auto x_ = x; \
--recur_depth; \
cerr << string(recur_depth, '\t') << "\e[91m" << __func__ << ":" \
<< __LINE__ << "\t" << #x << " = " << x_ << "\e[39m" << endl; \
}
#else
#define dbg(x)
#endif
using namespace std;
using namespace __gnu_pbds;
typedef pair<int, int> ii;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> llll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pair<int, int>> vii;
typedef vector<vii> vvii;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<pair<ll, ll>> vll;
typedef vector<vll> vvll;
typedef vector<bool> vb;
template <class type1>
using ordered_set = tree<type1, null_type, less<type1>, rb_tree_tag,
tree_order_statistics_node_update>;
template <typename A, typename B>
ostream &operator<<(ostream &os, const pair<A, B> &p) {
return os << '(' << p.first << ", " << p.second << ')';
}
template <typename T_container, typename T = typename enable_if<
!is_same<T_container, string>::value,
typename T_container::value_type>::type>
ostream &operator<<(ostream &os, const T_container &v) {
os << '{';
string sep;
for (const T &x : v)
os << sep << x, sep = ", ";
return os << '}';
}
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ll INF = 1e9 + 10;
const ld EPS = 1e-9;
const int SQRTN = 40;
// Saw Solution
void solve() {
int n, m; cin >> n >> m;
vii ds(m); for (auto &[b, p] : ds) cin >> b >> p;
vvi bds(n); for (auto &[b, p] : ds) bds[b].push_back(p);
for (int i=0; i<n; i++) sort(all(bds[i]));
int source = ds[0].first, target = ds[1].first;
vvi dist(n, vi(SQRTN, INF));
vector<vb> vis(n, vb(SQRTN));
dist[source][0] = 0;
typedef pair<int, ii> T;
priority_queue<T, vector<T>, greater<T>> pq; pq.push({0, {source, 0}});
auto add = [&](int b, int p, int d) {
if (dist[b][p] > d) {
dist[b][p] = d;
pq.push({d, {b, p}});
}
};
while (!pq.empty()) {
auto [dist, bbpp] = pq.top(); pq.pop();
auto [bb, pp] = bbpp;
if (vis[bb][pp]) continue;
vis[bb][pp] = 1;
add(bb, 0, dist);
if (pp == 0) {
for (auto np : bds[bb]) {
if (np < SQRTN) {
add(bb, np, dist);
continue;
}
for (int j = bb % np; j < n; j += np)
add(j, 0, dist + abs(bb - j) / np);
}
} else {
if (bb >= pp)
add(bb - pp, pp, dist + 1);
if (bb + pp < n)
add(bb + pp, pp, dist + 1);
}
}
cout << (dist[target][0] >= INF ? -1 : dist[target][0]) << "\n";
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(NULL);
clock_t start = clock();
int test_cases = 1;
/* cin >> test_cases; */
while (test_cases--)
solve();
#ifdef DEBUG
cerr << fixed << setprecision(10)
<< "\nTime Taken: " << (double)(clock() - start) / CLOCKS_PER_SEC
<< "s\n";
#endif
return 0;
}
