#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define pb push_back
#define all(x) begin(x), end(x)
#define SZ(x) (int)(x).size()
#define cps(x) sort(all(x)), (x).erase(unique(all(x)), end(x))
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int n0 = 2003, m0 = 2003;
const int k0 = n0 * m0;
int n, m, b[m0], p[m0], k, d[k0], inf, ind[n0][m0];
vector <array <int, 2>> g[k0];
//array <int, 2> t[k0];
void bfs(int x, int y) {
memset(& d, 0x3f, sizeof(d));
inf = d[0];
deque <int> q;
d[ind[x][y]] = 0;
q.push_front(ind[x][y]);
while (!q.empty()) {
int v = q.front();
q.pop_front();
for (auto & i : g[v]) {
int to = i[0], w = i[1];
if (d[to] > d[v] + w) {
d[to] = d[v] + w;
if (w == 0)
q.push_front(to);
else
q.pb(to);
}
}
}
}
int main() {
ios_base::sync_with_stdio(false), cin.tie(NULL);
cin >> n >> m;
for (int i = 0; i < m; i++)
cin >> b[i] >> p[i];
for (int i = 0; i < n; i++)
for (int j = 0; j <= m; j++) {
ind[i][j] = k++;
//t[k - 1] = {i, j};
}
for (int i = 0; i < m; i++)
g[ind[b[i]][m]].pb({ind[b[i]][i], 0});
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if ((i % p[j]) != (b[j] % p[j])) continue;
if (i - p[j] >= 0) g[ind[i][j]].pb({ind[i - p[j]][j], 1});
if (i + p[j] < n) g[ind[i][j]].pb({ind[i + p[j]][j], 1});
g[ind[i][j]].pb({ind[i][m], 0});
}
}
bfs(b[0], 0);
cout << d[ind[b[1]][1]];
}
