#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize("-Ofast")
#include <bits/stdc++.h>
#include <algorithm>
#include <iostream>
#include <vector>
#include <limits>
#include <cmath>
#include <stack>
#include <queue>
#include <map>
#include <math.h>
using namespace std;
using ll = long long;
void solve(){
ll n, m;
cin >> n >> m;
vector<vector<pair<ll, ll>>> adj(m, vector<pair<ll, ll>>());
vector<pair<ll, ll>> list_n(m, {0, 0});
for(ll i = 0; i < m; i++){
cin >> list_n[i].first;
cin >> list_n[i].second;
}
for(ll i = 0; i < m - 1; i++){
for(ll j = i + 1; j < m; j++){
if(abs(list_n[j].first - list_n[i].first)%list_n[i].second == 0){
adj[i].push_back({j, abs(list_n[j].first - list_n[i].first)/list_n[i].second});
}
if(abs(list_n[j].first - list_n[i].first)%list_n[j].second == 0){
adj[j].push_back({i, abs(list_n[j].first - list_n[i].first)/list_n[j].second});
}
}
}
multiset<pair<ll, ll>> mult;
vector<ll> vis(m, 1E17);
mult.insert({0, 0});
vis[0] = 0;
while(!mult.empty()){
auto top = *(mult.begin());
mult.erase(mult.begin());
for(auto node : adj[top.second]){
if(vis[node.first] <= top.first + node.second){
continue;
}else{
vis[node.first] = top.first + node.second;
mult.insert({top.first + node.second, node.first});
}
}
}
if(vis[1] == 1E17){
cout << -1 << endl;
}else{
cout << vis[1];
}
}
bool single = true;
signed main(){
ios_base::sync_with_stdio(false);
cout.tie(0);
cin.tie(0);
ll t = 1;
if(!single) cin >> t;
while(t--){
solve();
}
}
