#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<bitset<30002>> vis(30002);
vector<int> ap[n + 1];
int po, st, k1;
for (int i = 0; i < m; i++) {
int u, k;
cin >> u >> k;
ap[u].push_back(k);
if (i == 0) st = u, k1 = k;
if (i == 1) po = u;
}
int pa[2] = {-1, 1};
queue<array<int, 3>> qu;
qu.push({st, k1, 0});
vis[st][k1] = 1;
while (qu.size())
{
auto [u, k, p] = qu.front();
qu.pop();
//cout << "HERE";
//cout << u << ' ' << k << ' ' << p << '\n';
if (u == po) {
cout << p;
return 0;
}
for (auto& d : pa) {
int np = u - d * k;
//cout << "HERE";
if (np < 0 || np >= n || vis[np][k]) continue;
vis[np][k] = true;
//cout << "HERE";
qu.push({np, k, p + 1});
}
for (auto& o : ap[u]) {
for (auto& d : pa) {
int np = u - d * o;
if (np < 0 || np >= n || vis[np][o]) continue;
vis[np][o] = true;
qu.push({np, o, p + 1});
}
}
}
cout << -1;
return 0;
}
