#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <queue>
using namespace std;
/*
int n, m;
set<int> inc[30100];
vector< pair<int, int> > adj[2000000];
int dist[2000000];
bool vis[2000000];
priority_queue< pair<int, int> > ord;
int main () {
cin >> n >> m;
int rt = sqrt(n);
int start, end;
for (int i=0; i<n; i++) {
for (int j=1; j<rt; j++) {
adj[i*rt+j].push_back({i*rt, 0});
if (i-j >= 0) adj[i*rt+j].push_back({(i-j)*rt+j, 1});
if (i+j < n) adj[i*rt+j].push_back({(i+j)*rt+j, 1});
}
}
int bi, pi;
for (int i=0; i<m; i++) {
cin >> bi >> pi;
if (i == 0) start = bi;
if (i == 1) end = bi;
if (pi < rt) {
adj[bi*rt].push_back({bi*rt+pi, 0});
} else {
for (int j=1; j<n; j++) {
if (bi-j*pi < 0 && bi+j*pi >= n) break;
if (bi-j*pi >= 0) adj[bi*rt].push_back({(bi-j*pi)*rt, j});
if (bi+j*pi < n) adj[bi*rt].push_back({(bi+j*pi)*rt, j});
}
}
}
for (int i=0; i<n*rt; i++) {
dist[i] = 1e9;
vis[i] = false;
}
dist[start*rt] = 0;
ord.push({0, start*rt});
while (ord.size() > 0) {
pair<int, int> cur = ord.top();
cur.first = -cur.first;
ord.pop();
if (cur.second == end*rt) {
cout << cur.first << endl;
return 0;
}
if (vis[cur.second]) continue;
vis[cur.second] = true;
for (auto i: adj[cur.second]) {
if (!vis[i.first] && cur.first+i.second < dist[i.first]) {
dist[i.first] = cur.first+i.second;
ord.push({-dist[i.first], i.first});
}
}
}
cout << -1 << endl;
}
*/
int n, m;
set<int> inc[30100];
vector< pair<int, int> > adj[30100];
int dist[30100];
bool vis[30100];
priority_queue< pair<int, int> > pq;
int main () {
cin >> n >> m;
int bi, pi;
int start, end;
for (int i=0; i<m; i++) {
cin >> bi >> pi;
inc[bi].insert(pi);
if (i == 0) start = bi;
if (i == 1) end = bi;
}
for (int i=0; i<n; i++) {
for (auto j: inc[i]) {
for (int k=1; k<n; k++) {
if (i-j*k < 0) break;
if (inc[i-j*k].find(j) != inc[i-j*k].end()) {
adj[i].push_back({i-j*k, k});
break;
}
adj[i].push_back({i-j*k, k});
}
for (int k=1; k<n; k++) {
if (i+j*k >= n) break;
if (inc[i+j*k].find(j) != inc[i+j*k].end()) {
adj[i].push_back({i+j*k, k});
break;
}
adj[i].push_back({i+j*k, k});
}
}
}
for (int i=0; i<n; i++) {
dist[i] = 1e9;
}
dist[start] = 0;
pq.push({0, start});
while (pq.size() > 0) {
pair<int, int> cur = pq.top();
cur.first = -cur.first;
pq.pop();
if (cur.second == end) {
cout << cur.first << endl;
return 0;
}
if (vis[cur.second]) continue;
vis[cur.second] = true;
for (auto i: adj[cur.second]) {
if (!vis[i.first] && cur.first+i.second < dist[i.first]) {
dist[i.first] = cur.first+i.second;
pq.push({-dist[i.first], i.first});
}
}
}
cout << -1 << endl;
}
