#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 12;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<pair<int, int>> v(m);
for(auto &z : v) {
cin >> z.first >> z.second;
}
vector<vector<pair<int, int>>> kud(n);
for(auto &z : v) {
kud[z.first].push_back(z);
}
map<pair<int, int>, int> dist;
dist[v[0]] = 0;
deque<pair<int, int>> q;
q.push_back(v[0]);
int ans = INF;
while(!q.empty()) {
auto tren = q.front();
q.pop_front();
// cerr << tren.first << " " << tren.second << endl;
if(tren.first == v[1].first) {
ans = min(ans, dist[tren]);
break;
}
for(auto x : kud[tren.first]) {
if(!dist.count(x)) {
dist[x] = dist[tren];
q.push_front(x);
}
}
kud[tren.first].clear();
auto check = [&n](pair<int, int> x) {
return x.first >= 0 && x.first < n;
};
pair<int, int> levo{tren.first - tren.second, tren.second};
pair<int, int> desno{tren.first + tren.second, tren.second};
if(!dist.count(levo) && check(levo)) {
dist[levo] = dist[tren] + 1;
q.push_back(levo);
}
if(!dist.count(desno) && check(desno)) {
dist[desno] = dist[tren] + 1;
q.push_back(desno);
}
}
cout << ((ans >= INF) ? -1 : ans) << '\n';
}
