#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
#define int long long
const int N = 30069;
const int inf = 1e18;
int n, m, a[N], b[N], dis[N];;
vector<int> c[N];
vector<pair<int, 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];
c[a[i]].push_back(i);
}
for (int i = 1;i <= n;i++) {
dis[i] = inf;
}
priority_queue<tuple<int, int, int>> q;
q.push({0, a[1], 1});
dis[a[1]] = 0;
while (!q.empty()) {
auto v = get<2>(q.top()), h = get<1>(q.top());
q.pop();
for (int i = h, d = 0;i <= n;i += b[v], d++) {
if (dis[i] > dis[h] + d && !c[i].empty()) {
dis[i] = dis[h] + d;
for (auto u : c[i])
q.push({dis[i], i, u});
}
}
for (int i = h, d = 0;i >= 1;i -= b[v], d++) {
if (dis[i] > dis[h] + d && !c[i].empty()) {
dis[i] = dis[h] + d;
for (auto u : c[i])
q.push({dis[i], i, u});
}
}
}
if (dis[a[2]] == inf)
cout << -1 << '\n';
else
cout << dis[a[2]] << '\n';
}
return 0;
}
