#include <bits/stdc++.h>
using namespace std;
#define boost ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define fi first
#define se second
#define debug(x) cerr << " - " << #x << ": " << x << endl;
#define debugs(x, y) cerr << " - " << #x << ": " << x << " " << #y << ": " << y << endl;
#define debugii(x) cerr << " - " << #x << ": " << x.fi<<","<<x.se << endl;
#define sep() cerr << "--------------------" << endl;
#define all(x) (x).begin(),(x).end()
#define sz(x) (ll)x.size()
#define ll long long
#define ii pair<int,int>
#define v vector<int>
#define vii vector<ii>
#define vv vector<vector<int> >
#define mp make_pair
#define INF 100000000
#define pb push_back
#define EPS 1e-9
const int MOD = 1000000007; // 998244353
int B[30005],P[30005];
bool vis[30005],vis2[30005][30005];
int dist[30005];
vv vec;
int main(){
boost;
int N,M;
cin>>N>>M;
vec.resize(N);
for (int i = 0; i < M; ++i){
cin>>B[i]>>P[i];
vec[B[i]].pb(i);
}
vis[B[0]]=1;
queue<pair<int,ii> > q;
for (int i = 0; i < sz(vec[B[0]]); ++i)
q.push({0,{B[0],vec[B[0]][i]}});
while(!q.empty()){
pair<int,ii> front=q.front();
int curr=front.se.fi;
q.pop();
if(curr==B[1]){
cout<<front.fi<<"\n";
return 0;
}
int idx=curr+P[front.se.se];
if(idx<N){
if(!vis2[idx][front.se.se])
q.push({front.fi+1,{idx,front.se.se}}),vis2[idx][front.se.se]=1;
if(!vis[idx]){
for (int i = 0; i < sz(vec[idx]); ++i){
q.push({front.fi+1,{idx,vec[idx][i]}});
vis2[idx][vec[idx][i]]=1;
}
vis[idx]=1;
}
}
idx=curr-P[front.se.se];
if(idx>=0){
if(!vis2[idx][front.se.se])
q.push({front.fi+1,{idx,front.se.se}}),vis2[idx][front.se.se]=1;
if(!vis[idx]){
for (int i = 0; i < sz(vec[idx]); ++i){
q.push({front.fi+1,{idx,vec[idx][i]}});
vis2[idx][vec[idx][i]]=1;
}
vis[idx]=1;
}
}
}
cout<<-1;
cout<<"\n";
return 0;
}
//long long
//array bounds
//special cases
//binary search
