#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
const int nax = 30005;
const int INF = 1e9 + 5;
vector<int> dog[nax];
vector<int> jump(nax);
int dp[2005][nax];
int n, m;
int go(int sky, int doge) {
if (sky < 0 || sky >= n) return INF;
if (doge == 1) return 0;
int& ret = dp[sky][doge];
if (ret != -1) {
return ret;
}
ret = INF;
// pass news
for(int i : dog[sky]) {
ret = min(ret, go(sky, i));
}
// jump left
ret = min(ret, 1+go(sky-jump[doge], doge));
// jump left
ret = min(ret, 1+go(sky+jump[doge], doge));
return ret;
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
memset(dp, -1, sizeof dp);
cin >> n >> m;
int boss = 0;
for(int i = 0; i < m; i++) {
int idx, p;
cin >> idx >> p;
dog[idx].push_back(i);
jump[i] = p;
if (i == 0) {
boss = idx;
}
}
int ans = go(boss, 0);
if (ans >= INF) {
cout << -1 << "\n";
} else {
cout << ans << "\n";
}
return 0;
}
