#include <bits/stdc++.h>
using namespace std;
#define nl '\n'
using ll = long long;
const int inf = 1e9 + 5;
void solve() {
int n, m; cin >> n >> m;
vector<int> b(m), p(m);
vector<vector<int>> same(n);
for(int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
same[b[i]].push_back(i);
}
vector mod(n, vector(n, vector<int>()));
for(int i = 1; i < n; i++) {
for(int j = 0; j < m; j++) {
mod[i][b[j] % i].push_back(j);
}
}
vector<int> d(m, inf);
d[0] = 0;
queue<int> q;
q.push(0);
while(!q.empty()) {
int v = q.front();
q.pop();
if(p[v] == 0 || p[v] >= n) {
for(auto to : same[b[v]]) {
if(d[to] > d[v]) {
d[to] = d[v];
q.push(to);
}
}
} else {
for(auto to : mod[p[v]][b[v] % p[v]]) {
int cost = abs(b[to] - b[v]) / p[v];
if(d[to] > d[v] + cost) {
d[to] = d[v] + cost;
q.push(to);
}
}
}
}
if(d[1] == inf) d[1] = -1;
cout << d[1] << nl;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}