//#pragma GCC optimize(O3)
#include<bits/stdc++.h>
using namespace std;
#define L(i,j,k) for(int i=(j);i<=(k);i++)
#define R(i,j,k) for(int i=(j);i>=(k);i--)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
void solve(){
int n,m;
cin>>n>>m;
vector<vector<int>> adj(m);
vector<int> pos(m);
vector<int> pulo(m);
L(i,0,m-1)cin>>pos[i]>>pulo[i];
vector<vector<int>> dist(m,vector<int>(m));
L(i,0,m-1){
L(j,0,m-1){
if(i==j)continue;
dist[i][j]=(abs(pos[i]-pos[j])%pulo[i]);
if(dist[i][j]==0){dist[i][j]=abs((pos[i]-pos[j]))/pulo[i];adj[i].push_back(j);}
}
}
const int MOD=1e9+7;
vector<int> dp(m,MOD);
dp[0]=0;
priority_queue<pair<int,int>,vector<pair<int,int>>,greater<pair<int,int>>> pq;
pq.push({dp[0],0});
while(!pq.empty()){
auto [d,v]=pq.top();
pq.pop();
if(d!=dp[v])continue;
for(auto node:adj[v]){
if(dp[node]>dp[v]+dist[v][node]){
dp[node]=dp[v]+dist[v][node];
pq.push({dp[node],node});
}
}
}
cout<<((dp[1]==MOD)?-1:dp[1]);
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
solve();
}
//fazer um pseudo floyd warshal, na vdd da pra construir um super clique com as distancias de cada um
