#include <bits/stdc++.h>
using namespace std;
const int N = 30010, M = 30010;
int n, m, b[M], p[M], dist[N];
bool mark[N];
map<pair<int, int>, bool> seen;
vector<int> vec[N];
int main(){
cin >> n >> m;
for (int i = 0; i < m; i ++){
cin >> b[i] >> p[i];
vec[b[i]].push_back(p[i]);
}
for (int i = 0; i < n; i ++){
sort(vec[i].begin(), vec[i].end());
vec[i].resize(unique(vec[i].begin(), vec[i].end()) - vec[i].begin());
}
memset(dist, 63, sizeof dist);
dist[b[0]] = 0;
for (int ii = 0; ii < n; ii ++){
int v = -1;
for (int j = 0; j < n; j ++)
if (!mark[j] and (v == -1 or dist[v] > dist[j]))
v = j;
if (v == b[1]) break;
mark[v] = 1;
for (int d : vec[v]){
if (seen.find({v, d}) != seen.end()) continue;
for (int i = 1; v + d * i < n and dist[v] + i < dist[b[1]]; i ++){
if (seen.find({v + d * i, d}) != seen.end()) break;
if (dist[v + d * i] > dist[v] + i){
dist[v + d * i] = dist[v] + i;
seen[{v + d * i, d}] = 1;
}
}
for (int i = 1; v - d * i >= 0 and dist[v] + i < dist[b[1]]; i ++){
if (seen.find({v - d * i, d}) != seen.end()) break;
if (dist[v - d * i] > dist[v] + i){
dist[v - d * i] = dist[v] + i;
seen[{v - d * i, d}] = 1;
}
}
}
}
if (dist[b[1]] > 1e9)
dist[b[1]] = -1;
cout << dist[b[1]] << endl;
}
