#include <bits/stdc++.h>
#define fr first
#define sc second
#define pii pair<int, int>
#define pb push_back
#define szof(s) (int)s.size()
#define all(s) s.begin(), s.end()
#define fastInp ios_base::sync_with_stdio(0); cin.tie(0);
using namespace std;
const int MAXN = 30005;
const int SHIFT = 50000;
const int S = 200;
const int INF = 1e9;
int pos[MAXN], p[MAXN];
int n, m;
struct node {
int v, pw, cost;
node () {
}
node (int v_, int pw_, int cost_) {
v = v_;
pw = pw_;
cost = cost_;
}
};
struct Compare_node {
bool operator () (node const &p1, node const &p2) {
if (p1.cost != p2.cost) {
return p1.cost < p2.cost;
}
if (p1.v != p2.v) {
return p1.v < p2.v;
}
return p1.pw < p2.pw;
}
};
vector <int> big[MAXN];
vector <int> small[MAXN];
vector <node> g[MAXN][S];
int dist[MAXN][S];
bool in(int pos) {
return 0 <= pos && pos < n;
}
signed main() {
fastInp;
cin >> n >> m;
set <int> all_small;
for (int i = 0; i < m; i++) {
cin >> pos[i] >> p[i];
if (p[i] >= S) {
big[pos[i]].pb(p[i]);
} else {
small[pos[i]].pb(p[i]);
all_small.insert(p[i]);
}
}
// node -> v pw cost
for (int i = 0; i < n; i++) {
sort(all(small[i]));
small[i].erase(unique(all(small[i])), small[i].end());
for (int val : small[i]) {
g[i][0].pb(node(i, val, 0));
}
}
set <node, Compare_node> pq;
vector <bool> used(MAXN, false);
memset(dist, -1, sizeof(dist));
pq.insert(node(pos[0], 0, 0));
dist[pos[0]][0] = 0;
auto ers = [&](int v, int pw) {
auto it = pq.lower_bound(node(v, pw, -INF));
if (it != pq.end() && it->v == v && it->pw == pw) {
pq.erase(it);
}
};
while (!pq.empty()) {
int v = pq.begin()->v;
int pw = pq.begin()->pw;
int cost = pq.begin()->cost;
//cout << "! " << v << " " << pw << ": " << cost << endl;
pq.erase(pq.begin());
if (cost > dist[v][pw]) {
continue;
}
if (!used[v]) { // впервые добрались до v
used[v] = 1;
for (int el : big[v]) {
for (int to = v % el; to < n; to += el) {
int c = abs(v - to) / el;
if (dist[to][0] == -1 || dist[to][0] > cost + c) {
dist[to][0] = cost + c;
ers(to, 0);
pq.insert(node(to, 0, dist[to][0]));
}
}
}
}
// change our pw
if (dist[v][0] == -1 || dist[v][0] > dist[v][pw]) {
dist[v][0] = dist[v][pw];
ers(v, 0);
pq.insert(node(v, 0, dist[v][pw]));
}
if (pw == 0) {
for (int el : small[v]) {
int new_v = v;
int new_pw = el;
if (!in(new_v)) {
continue;
}
int c = 0;
if (dist[new_v][new_pw] == -1 || dist[new_v][new_pw] > dist[v][pw] + c) {
dist[new_v][new_pw] = dist[v][pw] + c;
ers(new_v, new_pw);
pq.insert(node(new_v, new_pw, dist[new_v][new_pw]));
}
}
}
for (int type = -1; type <= 1; type++) {
if (type == 0) {
continue;
}
int new_v = v + type * pw;
int new_pw = pw;
if (!in(new_v)) {
continue;
}
int c = 1;
if (dist[new_v][new_pw] == -1 || dist[new_v][new_pw] > dist[v][pw] + c) {
dist[new_v][new_pw] = dist[v][pw] + c;
ers(new_v, new_pw);
pq.insert(node(new_v, new_pw, dist[new_v][new_pw]));
}
}
}
cout << dist[pos[1]][0] << endl;
}
