#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <string>
#include <utility>
#include <vector>
#include <unordered_set>
#include <unordered_map>
using namespace std;
bool done[30000] = {};
int n, m, d0, d1, ans = -1;
unordered_set<int> doges[30000];
unordered_map<int, int> dists;
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.insert(make_pair(i, 30001));
}
dists[d0] = 0;
while (true){
int i = -1, md = 30001;
for (unordered_map<int, int>::iterator it = dists.begin(); it != dists.end(); it++){
if (it->second < md){
md = it->second; i = it->first;
}
}
if (i == -1){
break;
}
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);
}
}
}
dists.erase(i);
done[i] = true;
}
cout << ans << endl;
}
