#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
#define pb push_back
#define endl '\n'
using namespace std;
const int N = 3e4+5;
int b[N], p[N];
ll d[N];
vector<pii> adj[N];
int main() {
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n, m;
cin >> n >> m;
for (int i = 1; i <= m; i++) {
cin >> b[i] >> p[i];
d[i] = -1;
}
for (int i = 1; i <= m; i++)
for (int j = 1; j <= m; j++)
if (i != j && abs(b[i]-b[j]) % p[i] == 0)
adj[i].pb({j, abs(b[i]-b[j]) / p[i]});
priority_queue<pll, vector<pll>, greater<pll>> q;
q.push({0, 1});
while (q.size()) {
ll v = q.top().S, w = q.top().F;
q.pop();
if (d[v] != -1)
continue;
d[v] = w;
for (pii x : adj[v])
if (d[v] == -1)
q.push({w+x.S, x.F});
}
cout << d[2] << endl;
}
