#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
void solve(){
int n, m;
cin >> n >> m;
pair < int , int > doge[m];
for(int i = 0;i<m;i++){
cin >> doge[i].first >> doge[i].second;
}
int edge[m][m];
for(int i = 0;i<m;i++){
for(int j = 0;j<m;j++){
int len = abs(doge[i].first - doge[j].first);
if(len % doge[i].second == 0)edge[i][j] = len / doge[i].second;
else edge[i][j] = inf;
}
}
priority_queue < pair < int , int > , vector < pair < int , int > > , greater < pair < int , int > > > pq;
pq.push({0,0});
int vis[m];
memset(vis , 0 , sizeof(vis));
while(pq.size()){
pair < int , int > tmp = pq.top();
pq.pop();
if(vis[tmp.second])continue;
vis[tmp.second] = 1;
if(tmp.second == 1){
cout << tmp.first << endl;
return;
}
for(int i = 0;i<m;i++){
if(i != tmp.second and edge[tmp.second][i] != inf){
pq.push({tmp.first + edge[tmp.second][i] , i});
}
}
}
cout << -1 << endl;
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);
int testcase = 1;//cin >> testcase;
while(testcase--)solve();
}
