#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
const int N = 30069;
const int inf = 1e9;
int n, m, a[N], b[N], dis[N], was[N], o[N];
vector<int> g[N];
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int tt = 1;
//cin >> tt;
while (tt--) {
cin >> n >> m;
for (int i = 1;i <= m;i++) {
cin >> a[i] >> b[i], ++a[i];
g[a[i]].push_back(i);
}
for (int i = 1;i <= n;i++)
dis[i] = inf;
vector<int> vis;
vector<int> dogos;
dogos.push_back(1);
dis[a[1]] = 0;
while (dis[a[2]] == inf && !dogos.empty()) {
auto v = dogos.back();
dogos.pop_back();
for (int i = a[v], j = dis[a[v]];i <= n;i += b[v],j++) {
if (dis[i] == inf) {
dis[i] = j;
for (auto u : g[i])
dogos.push_back(u);
}
}
for (int i = a[v], j = dis[a[v]];i >= 1;i -= b[v],j++) {
if (dis[i] == inf) {
dis[i] = j;
for (auto u : g[i])
dogos.push_back(u);
}
}
}
if (dis[a[2]] == inf)
cout << -1 << '\n';
else
cout << dis[a[2]] << '\n';
}
return 0;
}
