#include <bits/stdc++.h>
using namespace std;
const int DIM = 30005;
int ans[DIM];
pair<int, int> arr[DIM];
vector<int> lst[DIM];
bitset<DIM> oki[DIM], vis;
struct str {
int ps, pw, tm; };
deque<str> que;
int main(void) {
#ifdef HOME
freopen("skyscraper.in", "r", stdin);
freopen("skyscraper.out", "w", stdout);
#endif
int n, m; cin >> n >> m;
for (int i = 0; i < m; ++i) {
cin >> arr[i].first >> arr[i].second;
lst[arr[i].first].push_back(i); ans[i] = 1000000000; }
que.push_back({arr[0].first, arr[0].second, 0});
ans[0] = 0; oki[arr[0].first][arr[0].second] = true;
while (que.size()) {
int ps = que.front().ps, pw = que.front().pw, tm = que.front().tm;
que.pop_front();
if (!vis[ps]) {
vis[ps] = true;
for (int x : lst[ps]) {
ans[x] = min(ans[x], tm);
if (!oki[arr[x].first][arr[x].second]) {
oki[arr[x].first][arr[x].second] = true;
que.push_front({arr[x].first, arr[x].second, tm}); } } }
if (ps + pw < n and !oki[ps + pw][pw]) {
oki[ps + pw][pw] = true;
que.push_back({ps + pw, pw, tm + 1}); }
if (ps - pw >= 0 and !oki[ps - pw][pw]) {
oki[ps - pw][pw] = true;
que.push_back({ps - pw, pw, tm + 1}); } }
if (ans[1] == 1000000000) {
ans[1] = -1; }
cout << ans[1];
return 0; }
