#include<bits/stdc++.h>
using namespace std;
const int nax = 3e4 + 3;
const int INF = 1e9;
int b[nax], p[nax];
vector<int>doges[nax];
map<pair<int,int>,int>dist;
int get(int x, int y) {
if(dist.count(make_pair(x, y))) return dist[{x, y}];
return INF;
}
void PlayGround() {
int n, m;
cin>>n>>m;
for(int i=0; i<m; ++i) {
cin>>b[i]>>p[i];
doges[b[i]].push_back(p[i]);
}
dist[{b[0], 0}] = 0;
queue<tuple<int,int,int>>q;
q.push({b[0], 0, 0});
while(!q.empty()) {
auto x = q.front();
q.pop();
int ss = get<0>(x);
int j = get<1>(x);
int d = get<2>(x);
if(d!=get(ss, j)) continue;
if(j==0) {
for(int z : doges[ss]) {
if(get(ss, z)>d) {
dist[{ss, z}] = d;
q.emplace(ss, z, d);
}
}
} else {
if(ss+j<n) {
if(get(ss+j, j)>d+1) {
dist[{ss+j, j}] = d+1;
q.emplace(ss+j, j, d+1);
}
if(get(ss+j, 0)>d+1) {
dist[{ss+j, 0}] = d+1;
q.emplace(ss+j, 0, d+1);
}
}
if(ss-j>=0) {
if(get(ss-j, j)>d+1) {
dist[{ss-j, j}] = d+1;
q.emplace(ss-j, j, d+1);
}
if(get(ss-j, 0)>d+1) {
dist[{ss-j, 0}] = d+1;
q.emplace(ss-j, 0, d+1);
}
}
}
}
int ans = get(b[1], 0);
if(ans==INF) ans = -1;
cout<<ans<<'\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
PlayGround();
return 0;
}
