#include "bits/stdc++.h"
using namespace std;
#define ll long long
#define endl '\n'
#define fi first
#define se second
#define vall(a) (a).begin(), (a).end()
#define sze(a) (ll)a.size()
#define pii pair<ll, ll>
#define pb push_back
#define pf push_front
const ll mod = 1e9 + 7;
const signed N = 30005;
const ll oo = 1e18;
ll n, m, p[N], b[N], d[N][200], base;
struct node {
ll idx, p, c;
bool operator<(const node &other) const {
return c > other.c;
}
};
vector<node> adj[N][200];
ll dijkstra() {
for (ll i = 0; i < n; ++i) {
for (ll j = 0; j <= base; ++j) d[i][j] = oo;
}
priority_queue<node> q;
d[b[0]][0] = 0;
q.push({b[0], 0, d[b[0]][0]});
while(!q.empty()) {
auto [idx, p, c] = q.top();
q.pop();
if (d[idx][p] != c) continue;
for (auto [pos, jump, w] : adj[idx][p]) {
if (d[pos][jump] > d[idx][p] + w) {
d[pos][jump] = d[idx][p] + w;
q.push({pos, jump, d[pos][jump]});
}
}
if (d[idx][0] > d[idx][p]) {
d[idx][0] = d[idx][p];
q.push({idx, 0, d[idx][0]});
}
if (p != 0) {
if (idx + p < n && d[idx + p][p] > d[idx][p] + 1) {
d[idx + p][p] = d[idx][p] + 1;
q.push({idx + p, p, d[idx + p][p]});
}
if (idx - p >= 0 && d[idx - p][p] > d[idx][p] + 1) {
d[idx - p][p] = d[idx][p] + 1;
q.push({idx - p, p, d[idx - p][p]});
}
}
}
return d[b[1]][0];
}
void output() {
for (ll i = 0; i < m; ++i) {
if (p[i] > base) {
for (ll j = 1; b[i] + p[i] * j < n; ++j) adj[b[i]][0].pb({b[i] + p[i] * j, 0, j});
for (ll j = 1; b[i] - p[i] * j >= 0; ++j) adj[b[i]][0].pb({b[i] - p[i] * j, 0, j});
} else {
adj[b[i]][p[i]].pb({b[i], 0, 0});
adj[b[i]][0].pb({b[i], p[i], 0});
}
}
cout << dijkstra();
return;
}
void input() {
cin >> n >> m;
base = sqrtl(n);
for (ll i = 0; i < m; ++i) cin >> b[i] >> p[i];
output();
return;
}
signed main () {
if(fopen("", "r")) {
freopen("", "r", stdin);
freopen("", "w", stdout);
}
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
input();
return 0;
}
