#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <string>
#include <utility>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <unordered_set>
using namespace std;
bool done[30001] = {};
long temp;
int n, m, d0, d1, dists[30001] = {}, ans = -1;
vector<int> doges[30001];
priority_queue<pair<int, int> > pq;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
for (int i = 0; i < m; i++){
int a, b; cin >> a >> b;
if (i == 0) d0 = a;
if (i == 1) d1 = a;
doges[a].push_back(b);
}
for (int i = 0; i < n; i++){
dists[i] = 30001;
}
pq.push(make_pair(0, d0));
dists[d0] = 0;
while (pq.size() > 0){
int i = pq.top().second; pq.pop();
if (done[i]){
continue;
}
if (i == d1){
ans = dists[i];
break;
}
for (int p : doges[i]){
for (int j=i+p, add = 1; j<n; j+= p, add++) {
temp = dists[i]+add;
if (temp < dists[j]) {
dists[j] = temp;
pq.push(make_pair(-dists[j], j));
}
}
for (int j=i-p, add = 1; j>= 0; j-= p, add++) {
temp = dists[i]+add;
if (temp < dists[j]) {
dists[j] = temp;
pq.push(make_pair(-dists[j], j));
}
}
}
done[i] = true;
}
cout << ans << endl;
}
