#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
#include <random>
#include <chrono>
#include <tuple>
#include <random>
#include <cmath>
#include <unordered_set>
#include <bitset>
using namespace std;
typedef long long ll;
typedef long double ld;
#define fastInp cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
const ll SZ = 30002, INF = 1e9 + 10;
ll n, m;
unordered_set<ll> us;
ll dist[SZ];
bool used[SZ];
vector<ll> cur[SZ];
bitset<SZ> bts[SZ];
int main()
{
fastInp;
cin >> n >> m;
ll ind = 0, ind2 = 0, b2 = 0;
for (int i = 0; i < m; i++) {
ll b, p;
cin >> b >> p;
cur[b].push_back(p);
if (i == 0) {
ind2 = b;
b2 = p;
}
if (i == 1) {
ind = b;
}
}
queue<pair<ll, ll>> q;
queue<ll> q2;
q.push({ ind2, b2 });
for (int i = 0; i < n; i++) dist[i] = INF;
dist[ind2] = 0;
q2.push(0);
swap(n, m);
while (!q.empty()) {
ll v = q.front().first;
pair<ll, ll> u = q.front();
ll d = q2.front();
q2.pop();
q.pop();
ll c = u.second;
if (v - c >= 0 && v - c < m && !bts[v - c][c]) {
if (v - c >= 0 && v - c < m) {
bts[v - c][c] = 1;
dist[v - c] = min(dist[v - c], d + 1);
q.push({ v - c, c });
q2.push({ d + 1 });
}
}
if (v + c >= 0 && v + c < m && !bts[v + c][c]) {
if (v + c >= 0 && v + c < m) {
bts[v + c][c] = 1;
dist[v + c] = min(dist[v + c], d + 1);
q.push({ v + c, c });
q2.push({ d + 1 });
}
}
if (!used[v]) {
used[v] = 1;
for (auto c : cur[v]) {
if (v - c >= 0 && v - c < m && !bts[v - c][c]) {
if (v - c >= 0 && v - c < m) {
bts[v - c][c] = 1;
dist[v - c] = min(dist[v - c], d + 1);
q.push({ v - c, c });
q2.push({ d + 1 });
}
}
if (v + c >= 0 && v + c < m && !bts[v + c][c]) {
if (v + c >= 0 && v + c < m) {
bts[v + c][c] = 1;
dist[v + c] = min(dist[v + c], d + 1);
q.push({ v + c, c });
q2.push({ d + 1 });
}
}
}
}
if (dist[ind] != INF) {
cout << dist[ind];
return 0;
}
}
cout << "-1";
return 0;
}
/*
5
1 4 1 3 5
2 1 4 1 2
5 1 1 4 5
3 5 1 1 2
4 3 5 1 1
3
1 1 1
2 2 2
3 3 3
*/
