#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
const int N = 3e4 + 5;
const int oo = 1e9;
const int SQRT = 175;
#define eb emplace_back
struct node {
int w, u, dir;
node() {
w = u = dir = 0;
}
node(int w, int u, int dir) : w(w), u(u), dir(dir) {}
friend bool operator > (node a, node b) {
return a.w > b.w;
}
};
int n, m;
int dist[N][SQRT];
pii a[N];
vector<int> adj[N];
priority_queue<node, vector<node>, greater<node>> Q;
void update(int u, int v, int w, int dir,int pre) {
if(dist[v][dir] > dist[u][pre] + w) {
dist[v][dir] = dist[u][pre] + w;
Q.emplace(dist[v][dir], v, dir);
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++) {
int u, v;
cin >> u >> v;
adj[u].eb(v);
a[i] = {u, v};
}
memset(dist, 0x3f, sizeof dist);
dist[a[0].first][0] = 0;
Q.emplace(0, a[0].first, 0);
while(!Q.empty()) {
node top = Q.top();
Q.pop();
int u = top.u;
int dir = top.dir;
if(top.w != dist[u][dir])
continue;
//cout << u << ' ' << dir << '\n';
if(dir) {
if(u + dir <= n)
update(u, u + dir, 1, dir, dir);
if(u - dir >= 0)
update(u, u - dir, 1, dir, dir);
update(u, u, 0, 0 , dir);
}
for(int v : adj[u]) {
if(v >= SQRT) {
for(int j = 1; u + j * v <= n; j++)
update(u, u + j * v, j, 0, 0);
for(int j = 1; u - j * v >= 0; j++)
update(u, u - j * v, j, 0, 0);
} else {
if(u + v <= n)
update(u, u + v, 1, v , 0);
if(u - v >= 0)
update(u, u - v, 1, v , 0);
}
}
}
int res = 1e9;
for (int i = 0 ; i < SQRT; ++i) {
res = min(res , dist[a[1].first][i]);
// cout << dist[1][i] << '\n';
}
cout << res;
}
/*
5 3
0 2
1 1
4 1
*/
