#include <bits/stdc++.h>
using namespace std;
#define int long long
priority_queue<pair<int,int>> q;
vector<int> buildings[30001];
int power[30001];
set<int> visited;
int n;
inline void adddoge(int x,int building,int dist){
for(int i:visited)if(abs(building-i)%power[x]==0){
q.emplace(dist-(abs(building-i)/power[x]),i);
}
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int m;
cin >> n >> m;
int base,tar;
cin>>base>>power[0]>>tar>>power[1];
buildings[base].emplace_back(0);
buildings[tar].emplace_back(1);
for(int i=2;i<m;i++){
int b;cin>>b>>power[i];
buildings[b].emplace_back(i);
}
for(int i=0;i<n;i++)visited.insert(i);
q.emplace(0,base);
while(!q.empty()){
auto curr = q.top();q.pop();
if(visited.count(curr.second)==0)continue;
visited.erase(curr.second);
if(curr.second==tar){
cout << -curr.first << '\n';
return 0;
}
for(int&i:buildings[curr.second])adddoge(i,curr.second,curr.first);
}
cout << "-1\n";
}
