#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
const short MAX_N = 30000;
const int MAX_W = 7350000;
short qb[MAX_W], qp[MAX_W];
short B[MAX_N], P[MAX_N];
gp_hash_table <short, int> dist[MAX_N];
vector <short> graph[MAX_N];
int main() {
short N, M;
cin >> N >> M;
for(short i = 0; i < M; i++) {
cin >> B[i] >> P[i];
graph[B[i]].push_back(P[i]);
}
for(short i = 0; i < N; i++) {
sort(graph[i].begin(), graph[i].end());
graph[i].resize(unique(graph[i].begin(), graph[i].end()) - graph[i].begin());
}
int l = 0, r = 0;
qb[r] = B[0];
qp[r++] = P[0];
dist[B[0]][P[0]] = 0;
while(l < r) {
short b = qb[l], p = qp[l++];
if(b == B[1]) {
cout << dist[b][p];
return 0;
}
if(b - p >= 0 and dist[b - p].find(p) == dist[b - p].end()) {
dist[b - p][p] = dist[b][p] + 1;
qb[r] = b - p;
qp[r++] = p;
}
if(b + p < N and dist[b + p].find(p) == dist[b + p].end()) {
dist[b + p][p] = dist[b][p] + 1;
qb[r] = b + p;
qp[r++] = p;
}
for(short p2 : graph[b]) {
if(b - p2 >= 0 and dist[b - p2].find(p2) == dist[b - p2].end()) {
dist[b - p2][p2] = dist[b][p] + 1;
qb[r] = b - p2;
qp[r++] = p2;
}
if(b + p2 < N and dist[b + p2].find(p2) == dist[b + p2].end()) {
dist[b + p2][p2] = dist[b][p] + 1;
qb[r] = b + p2;
qp[r++] = p2;
}
}
}
cout << -1;
return 0;
}
