//#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
#define int long long
priority_queue<pair<int,pair<int,int>>> q;
vector<int> buildings[30001];
int power[30001];
bool visited[30000][201];
const int INF = 1e15;
int n;
int threshold = 200;
inline void adddoge(int x,int building,int dist){
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int m;
cin >> n >> m;
threshold = sqrt(n);
int base,tar;
cin>>base>>power[0]>>tar>>power[1];
buildings[base].emplace_back(power[0]);
buildings[tar].emplace_back(power[1]);
for(int i=2;i<m;i++){
int b;cin>>b>>power[i];
buildings[b].emplace_back(power[i]);
}
q.emplace(0, make_pair(base,0));
while(!q.empty()){
auto curr = q.top();q.pop();
if(visited[curr.second.first][curr.second.second])continue;
visited[curr.second.first][curr.second.second]=true;
if(curr.second.first==tar){
cout << -curr.first << '\n';
return 0;
}
if(curr.second.second==0)for(int&i:buildings[curr.second.first]) {
if(i>threshold){
for (int j = 1; curr.second.first-j*i>= 0; ++j) {
int posnext = curr.second.first - j*i;
if (!visited[posnext][0])q.emplace(curr.first-j, make_pair(posnext, 0));
}
for (int j = 1; curr.second.first+j*i < n; ++j) {
int posnext = curr.second.first + j*i;
if (!visited[posnext][0])q.emplace(curr.first-j, make_pair(posnext, 0));
}
} else {
if(!visited[curr.second.first][i])q.emplace(curr.first,make_pair(curr.second.first,i));
}
}
else {
if(!visited[curr.second.first][0])q.emplace(curr.first, make_pair(curr.second.first,0));
if(curr.second.first+curr.second.second<n and !visited[curr.second.first+curr.second.second][curr.second.second])q.emplace(curr.first-1,make_pair(curr.second.first+curr.second.second,curr.second.second));
if(curr.second.first-curr.second.second>=0 and !visited[curr.second.first-curr.second.second][curr.second.second])q.emplace(curr.first-1,make_pair(curr.second.first-curr.second.second,curr.second.second));
}
}
cout << "-1\n";
}
