#include <bits/stdc++.h>
using namespace std;
#define int long long
priority_queue<pair<int,int>> q;
vector<pair<int,int>> adj[6000000];
bool visited[6000000];
int n;
const int threshold = 173;
void adddoge(int power,int building){
if(power>threshold){
int minimum = building - power*(building/power);
for (int i = minimum; i < n; i += power)
adj[building*174].emplace_back(i*174,abs(building-i)/power);
} else {
adj[building*174].emplace_back(building*174+power,0);
}
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int m;
cin >> n >> m;
int base,tar,bp,tp;
cin>>base>>bp>>tar>>tp;
adddoge(bp,base);
adddoge(tp,tar);
for(int i=2;i<m;i++){
int b,p;cin>>b>>p;
adddoge(p,b);
}
for(int i=0;i<n;i++){
int b = i*174;
for(int j=b+1;j<b+174;j++){
adj[j].emplace_back(b,0);
}
}
for(int power=1;power<=173;power++){
int offset = 174*power;
for(int i=power;i<n;i++){
adj[i*174+power].emplace_back(i*174+power-offset,1);
}
for(int i=n-power-1;i>=0;i--){
adj[i*174+power].emplace_back(i*174+power+offset,1);
}
}
base*=174;tar*=174;
q.emplace(0, base);
while(!q.empty()){
auto curr = q.top();q.pop();
if(visited[curr.second])continue;
visited[curr.second]=true;
if(curr.second==tar){
cout << -curr.first << '\n';
return 0;
}
for(auto&i:adj[curr.second])if(!visited[i.first])q.emplace(curr.first-i.second,i.first);
}
cout << "-1\n";
}
