#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <bitset>
#include <fstream>
using namespace std;
#define ii pair <int, int>
#define app push_back
#define all(a) a.begin(), a.end()
#define bp __builtin_popcountll
#define ll long long
#define mp make_pair
#define x first
#define y second
#define Time (double)clock()/CLOCKS_PER_SEC
#define debug(x) std::cerr << #x << ": " << x << '\n';
#define FOR(i, n) for (int i = 0; i < n; ++i)
#define FORN(i, n) for (int i = 1; i <= n; ++i)
#define pb push_back
#define trav(a, x) for (auto& a : x)
using vi = vector<int>;
template <typename T>
std::istream& operator >>(std::istream& input, std::vector<T>& data)
{
for (T& x : data)
input >> x;
return input;
}
template <typename T>
std::ostream& operator <<(std::ostream& output, const pair <T, T> & data)
{
output << "(" << data.x << "," << data.y << ")";
return output;
}
template <typename T>
std::ostream& operator <<(std::ostream& output, const std::vector<T>& data)
{
for (const T& x : data)
output << x << " ";
return output;
}
ll div_up(ll a, ll b) { return a/b+((a^b)>0&&a%b); } // divide a by b rounded up
ll div_down(ll a, ll b) { return a/b-((a^b)<0&&a%b); } // divide a by b rounded down
ll math_mod(ll a, ll b) { return a - b * div_down(a, b); }
#define tcT template<class T
#define tcTU tcT, class U
tcT> using V = vector<T>;
tcT> void re(V<T>& x) {
trav(a, x)
cin >> a;
}
tcT> bool ckmin(T& a, const T& b) {
return b < a ? a = b, 1 : 0;
} // set a = min(a,b)
tcT> bool ckmax(T& a, const T& b) {
return a < b ? a = b, 1 : 0;
}
ll gcd(ll a, ll b) {
while (b) {
tie(a, b) = mp(b, a % b);
}
return a;
}
const int N = 30007;
int n, m;
int dist[N];
vi have[N];
int pos[N], step[N];
bool used[N];
const int K = 200;
bool vis[K][N];
signed main() {
#ifdef LOCAL
#else
#define endl '\n'
ios_base::sync_with_stdio(0); cin.tie(0);
#endif
FOR (i, N) {
dist[i] = N;
}
cin >> n >> m;
FOR (i, m) {
cin >> pos[i] >> step[i];
have[pos[i]].app(step[i]);
}
dist[pos[0]] = 0;
FOR (t, n) {
int u = -1;
FOR (i, n) {
if (!used[i] && dist[i] < N) {
if (u == -1 || dist[i] < dist[u]) {
u = i;
}
}
}
if (u == -1) {
break;
}
used[u] = 1;
trav (d, have[u]) {
for (int i = 1; i * d <= u; ++i) {
if (d < K) {
if (vis[d][u - i * d]) {
break;
}
vis[d][u - i * d] = 1;
}
ckmin(dist[u - i * d], dist[u] + i);
}
for (int i = 1; u + i * d < n; ++i) {
if (d < K) {
if (vis[d][u + i * d]) {
break;
}
vis[d][u + i * d] = 1;
}
ckmin(dist[u + i * d], dist[u] + i);
}
}
}
if (dist[pos[1]] == N) {
cout << -1 << endl;
}
else {
cout << dist[pos[1]] << endl;
}
}
