//thatsramen
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define eb emplace_back
#define pb push_back
#define ft first
#define sd second
#define pi pair<int, int>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define dbg(...) dbg_out(__VA_ARGS__)
using ll = long long;
using ld = long double;
using namespace std;
using namespace __gnu_pbds;
//Constants
const ll INF = 5 * 1e18;
const int IINF = 2 * 1e9;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ll dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
const ld PI = 3.14159265359;
//Templates
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 << ']';}
void dbg_out() {cerr << endl;}
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr << H << ' '; dbg_out(T...); }
template<typename T> void mins(T& x, T y) {x = min(x, y);}
template<typename T> void maxs(T& x, T y) {x = max(x, y);}
template<typename T> using oset = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T> using omset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;
//order_of_key(k): number of elements strictly less than k
//find_by_order(k): k-th element in the set
void setPrec() {cout << fixed << setprecision(15);}
void unsyncIO() {cin.tie(0)->sync_with_stdio(0);}
void setIn(string s) {freopen(s.c_str(), "r", stdin);}
void setOut(string s) {freopen(s.c_str(), "w", stdout);}
void setIO(string s = "") {
unsyncIO(); setPrec();
if(s.size()) setIn(s + ".in"), setOut(s + ".out");
}
// #define TEST_CASES
const int sq = 170;
void solve() {
int n, m;
cin >> n >> m;
vector<vector<pi>> e(n);
vector<set<int>> ps(n);
pi eds;
for (int i = 0; i < m; i++) {
int st, p; cin >> st >> p;
if (i == 0) eds.ft = st;
if (i == 1) eds.sd = st;
if (p > sq) {
for (int j = st + p, dd = 1; j < n; j += p, dd++) {
e[st].pb({j, dd});
}
for (int j = st - p, dd = 1; j >= 0; j -= p, dd++) {
e[st].pb({j, dd});
}
} else {
ps[st].insert(p);
}
}
priority_queue<tuple<int, int, int>, vector<tuple<int, int, int>>, greater<tuple<int, int, int>>> que;
vector<vector<int>> dist(n, vector<int> (sq + 10, IINF));
vector<vector<bool>> vis(n, vector<bool> (sq + 10, false));
dist[eds.ft][0] = 0;
que.push({0, eds.ft, 0});
while (!que.empty()) {
auto [_, v, prev] = que.top();
que.pop();
if (vis[v][prev]) continue;
vis[v][prev] = true;
if (!prev) {
for (auto [to, cost] : e[v]) {
if (dist[to][prev] > dist[v][prev] + cost) {
dist[to][prev] = dist[v][prev] + cost;
que.push({dist[to][prev], to, prev});
}
}
for (int p : ps[v]) {
if (!vis[v][p]) {
dist[v][p] = dist[v][prev];
que.push({dist[v][prev], v, p});
}
}
} else {
if (!vis[v][0]) {
dist[v][0] = dist[v][prev];
que.push({dist[v][prev], v, 0});
}
if (v + prev < n) {
int to = v + prev;
if (dist[to][prev] > dist[v][prev] + 1) {
dist[to][prev] = dist[v][prev] + 1;
que.push({dist[to][prev], to, prev});
}
}
if (v - prev >= 0) {
int to = v - prev;
if (dist[to][prev] > dist[v][prev] + 1) {
dist[to][prev] = dist[v][prev] + 1;
que.push({dist[to][prev], to, prev});
}
}
}
}
int ans = IINF;
for (int i = 0; i <= sq; i++) {
mins(ans, dist[eds.sd][i]);
}
cout << ans;
}
int main() {
setIO();
int tt = 1;
#ifdef TEST_CASES
cin >> tt;
#endif
while (tt--) {
solve();
if (tt) {
cout << '\n' << '\n';
}
}
return 0;
}
