// IN THE NAME OF GOD
#include<bits/stdc++.h>
using namespace std;
typedef long long int lli;
typedef pair<lli,lli> pii;
typedef vector<lli> ve;
typedef vector<pii> vp;
const lli N = 5e5 + 100;
const lli INF = 1e18;
const lli mod = 1e9 + 7;
#define PB push_back
#define MP make_pair
#define fi first
#define se second
#define FOR(x,y) for(lli x = 0; x < y; x ++)
#define FORS(x,y) for(lli x = 1; x <= y; x ++)
#define all(x) x.begin(),x.end()
#define lb lower_bound
#define ub upper_bound
#define debug(x) cerr<<(#x)<<" "<<x<<endl
lli n,m;
lli B[N],P[N];
vp adj[N];
lli dis[N];
int main(){
ios_base::sync_with_stdio(false),cin.tie(0),cout.tie(0);
cin>>n>>m;
FOR(i,m){
cin>>B[i]>>P[i];
}
FOR(a,m){
FOR(b,m){
lli fa = abs(B[b]-B[a]);
if(a != b && fa%P[a] == 0){
adj[a].PB(MP(b,fa/P[a]));
adj[b].PB(MP(b,2*(fa/P[a]) ));
}
}
}
FOR(i,m)
dis[i] = INF;
priority_queue<pii,vp,greater<pii>> pq;
pq.push(MP(0,0));
dis[0] = 0;
while(pq.size()){
lli v = pq.top().se;
lli D = pq.top().fi;
pq.pop();
if(D != dis[v])continue;
for(auto [u,w] : adj[v]){
if(dis[u] > dis[v] + w){
dis[u] = dis[v] + w;
pq.push(MP(dis[u],u));
}
}
}
if(dis[1] == INF)dis[1] = -1;
cout<<dis[1]<<endl;
}
