#include "bits/stdc++.h"
using namespace std;
using ll = long long;
const int maxn=3e4+5;
const int B = 180;
int n,m,st,fn;
int dis[maxn * B];
bool vis[maxn * B];
vector<pair<int,int>>E[maxn * B];
int main(){
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
int sq = sqrt(n)+1;
for(int i = 0;i < n;i++){
for(int j = 1;j < sq;j++){
E[i * sq + j].push_back({i * sq,0});
if(i - j >= 0) E[i * sq + j].push_back({(i - j) * sq + j,1});
if(i + j < n) E[i * sq + j].push_back({(i + j) * sq + j,1});
}
}
for(int i = 0;i < m;i++){
int b,p; cin >> b >> p;
if(i == 0) st = b;
if(i == 1) fn = b;
if(p < sq) E[b * sq].push_back({b * sq + p,0});
else {
for(int j = 1;b - j * p >= 0;j++) E[b * sq].push_back({(b - j * p) * sq,j});
for(int j = 1;b + j * p < n;j++) E[b * sq].push_back({(b + j * p) * sq,j});
}
}
for(int i = 0;i < n * sq;i++) dis[i] = 1e9;
// cout<<st<<' '<<fn<<'\n';
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;
// cout<<x<<'\n';
for(auto i : E[x]){
if(dis[i.first] > dis[x] + i.second){
dis[i.first] = dis[x] + i.second;
q.push({-dis[i.first],i.first});
}
}
}
if(dis[fn * sq] == 1e9) cout<<-1;
else cout<<dis[fn * sq];
}
