#include<bits/stdc++.h>
using namespace std;
#define f first
#define s second
#define int long long
const int N = 2010;
const int M = 30010;
set<pair<int, int>> g[N];
int n, m, a[M], b[M], d[N];
signed main() {
ios_base::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 <= n;i++)
d[i] = 1e9;
for (int i = 1;i <= m;i++) {
cin >> a[i] >> b[i];
++a[i];
}
for (int i = 1;i <= m;i++) {
for (int j = a[i] + b[i], k = 1;j <= n;j += b[i],k++)
g[a[i]].insert({j, k});
for (int j = a[i] - b[i], k = 1;j >= 1;j -= b[i],k++)
g[a[i]].insert({j, k});
}
priority_queue<pair<int, int>> pq;
pq.push({0, 1});
d[1] = 0;
while (!pq.empty()) {
auto v = pq.top().s;
pq.pop();
for (auto u : g[v]) {
if (d[u.f] <= d[v] + u.s)
continue;
d[u.f] = d[v] + u.s;
pq.push({-d[u.f], u.f});
}
}
if (d[2] >= 1e9)
cout << -1;
else
cout << d[2];
}
return 0;
}
