#pragma optimize("Bismillahirrahmanirrahim")
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//Allahuekber
//ahmet23 orz...
//Sani buyuk Osman Pasa Plevneden cikmam diyor.
//FatihSultanMehmedHan
//YavuzSultanSelimHan
//AbdulhamidHan
#define author tolbi
#include <bits/stdc++.h>
using namespace std;
template<typename X, typename Y> istream& operator>>(istream& in, pair<X,Y> &pr) {return in>>pr.first>>pr.second;}
template<typename X, typename Y> ostream& operator<<(ostream& os, pair<X,Y> pr) {return os<<pr.first<<" "<<pr.second;}
template<typename X> istream& operator>>(istream& in, vector<X> &arr) {for(auto &it : arr) in>>it; return in;}
template<typename X> ostream& operator<<(ostream& os, vector<X> arr) {for(auto &it : arr) os<<it<<" "; return os;}
template<typename X, size_t Y> istream& operator>>(istream& in, array<X,Y> &arr) {for(auto &it : arr) in>>it; return in;}
template<typename X, size_t Y> ostream& operator<<(ostream& os, array<X,Y> arr) {for(auto &it : arr) os<<it<<" "; return os;}
#define int long long
#define endl '\n'
#define vint(x) vector<int> x
#define deci(x) int x;cin>>x;
#define decstr(x) string x;cin>>x;
#define cinarr(x) for (auto &it : x) cin>>it;
#define coutarr(x) for (auto &it : x) cout<<it<<" ";cout<<endl;
#define sortarr(x) sort(x.begin(),x.end())
#define sortrarr(x) sort(x.rbegin(),x.rend())
#define det(x) cout<<"NO\0YES"+x*3<<endl;
#define INF LONG_LONG_MAX
#define rev(x) reverse(x.begin(),x.end());
#define ios ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define tol(bi) (1LL<<((int)(bi)))
const int MOD = 1e9+7;
mt19937 ayahya(chrono::high_resolution_clock().now().time_since_epoch().count());
int32_t main(){
ios;
int t=1;
int tno = 0;
if (!t) cin>>t;
while (t-(tno++)){
deci(n);deci(m);
vector<vector<int>> arr(n);
int bas = -1;
int son = -1;
for (int i = 0; i < m; i++){
deci(x);deci(p);
if (i==0) bas = x;
if (i==1) son = x;
arr[x].push_back(p);
}
vector<int> sp(n,-1);
queue<array<int,4>> q;//nerde, kacla yuruo, time, nereye gidio
sp[bas]=0;
for (int i = 0; i < arr[bas].size(); ++i)
{
if (bas-arr[bas][i]>=0){
q.push({bas-arr[bas][i],arr[bas][i],1,0});
}
if (bas+arr[bas][i]<n){
q.push({bas+arr[bas][i],arr[bas][i],1,1});
}
}
while (q.size()){
int node = q.front()[0];
int step = q.front()[1];
int tim = q.front()[2];
int wh = q.front()[3];
q.pop();
if (sp[node]==-1){
sp[node]=tim;
if (node==son) break;
for (int i = 0; i < arr[node].size(); i++){
if (arr[node][i]==step) continue;//optimize etcez
if (node-arr[node][i]>=0){
q.push({node-arr[node][i],arr[node][i],tim+1,0});
}
if (node+arr[node][i]<n){
q.push({node+arr[node][i],arr[node][i],tim+1,1});
}
}
}
if (wh==0){
if (node-step>=0) q.push({node-step,step,tim+1,wh});
}
else {
if (node+step<n) q.push({node+step,step,tim+1,wh});
}
}
cout<<sp[son]<<endl;
}
}
