#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[30000] = {};
int n, m, d0, d1, dists[30000] = {}, ans = -1;
unordered_set<int> doges[30000];
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].insert(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 (long ni = i % p; ni < n; ni+=p){
if (!done[ni] && dists[ni] > dists[i] + abs((i-ni)/p)){
dists[ni] = dists[i] + abs((i-ni)/p);
pq.push(make_pair(-1 * dists[ni], ni));
}
}
}
done[i] = true;
}
cout << ans << endl;
}
