#include "bits/stdc++.h"
using namespace std;
using ll = long long;
const int maxn=3e4+5;
const int B = 192;
int n,m,st,fn;
int b[maxn],p[maxn];
int dis[maxn * B];
bool vis[maxn * B],done[maxn];
vector<int>doge[maxn];
int main(){
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
int sq = sqrt(n)+1;
for(int i = 0;i < m;i++){
cin >> b[i] >> p[i];
if(i == 0) st = b[i];
if(i == 1) fn = b[i];
doge[b[i]].push_back(p[i]);
}
for(int i = 0;i < n * sq;i++) dis[i] = 1e9;
dis[st * sq] = 0;
priority_queue<pair<int,int>>q;
q.push({0,st * sq});
while(!q.empty()){
int x = q.top().second;
q.pop();
if(vis[x]) continue;
vis[x] = 1;
int bb = x / sq;
int pw = x % sq;
// cout<<bb<<' '<<pw<<'\n';
if(!pw){
for(auto i : doge[bb]){
if(i < sq){
int to = bb * sq + i;
if(dis[to] > dis[x]){
dis[to] = dis[x];
q.push({-dis[to],to});
}
}
else {
for(int j = 1;;j++){
int to = bb - i * j;
if(to < 0) break;
if(dis[to * sq] > dis[x] + j){
dis[to * sq] = dis[x] + j;
q.push({-dis[to * sq],to * sq});
}
}
for(int j = 1;;j++){
int to = bb + i * j;
if(to >= n) break;
if(dis[to * sq] > dis[x] + j){
dis[to * sq] = dis[x] + j;
q.push({-dis[to * sq],to * sq});
}
}
}
}
}
else {
if(dis[bb * sq] > dis[x]){
dis[bb * sq] = dis[x];
q.push({-dis[bb*sq],bb*sq});
}
if(bb - pw >= 0 and dis[(bb - pw) * sq + pw] > dis[x] + 1){
dis[(bb - pw) * sq + pw] = dis[x] + 1;
q.push({-dis[(bb - pw) * sq + pw],(bb - pw) * sq + pw});
}
if(bb + pw < n and dis[(bb + pw) * sq + pw] > dis[x] + 1){
dis[(bb + pw) * sq + pw] = dis[x] + 1;
q.push({-dis[(bb + pw) * sq + pw],(bb + pw) * sq + pw});
}
}
}
if(dis[fn * sq] == 1e9) cout<<-1;
else cout<<dis[fn * sq];
}
