#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
typedef pair<int,int> pii;
const int MAXN = 3e4;
bitset <30000> bs[MAXN];
int main(){
ll n, m; cin >> n >> m;
vector<ll> b(m + 5), p(m + 5);
for(int i = 1; i <= m; i++) cin >> b[i] >> p[i];
vector<ll> adj[n + 5];
for(int i = 1; i <= m; i++){
adj[b[i]].push_back(p[i]);
}
vector<ll> pq[2];
pq[0].push_back(b[1]);
for(int dist=0;!pq[dist&1].empty();dist++){
for (auto idx : pq[dist&1]) {
if (idx == b[2]) {
cout << dist << "\n";
return 0;
}
for(auto i : adj[idx]){
// cout << idx << " " << i << "\n";
bs[idx][i] = 1;
if(idx+i < n && !bs[idx+i][i]){
bs[idx+i][i] = 1;
pq[dist&1^1].push_back(idx+i);
adj[idx+i].push_back(i);
}
if(idx-i >= 0 && !bs[idx-i][i]){
bs[idx-i][i] = 1;
pq[dist&1^1].push_back(idx-i);
adj[idx-i].push_back(i);
}
}
adj[idx].clear();
}
pq[dist&1].clear();
}
cout << "-1\n";
}
