#include <bits/stdc++.h>
#pragma GCC optimize ("O2")
#pragma GCC optimize ("unroll-loops")
//#pragma GCC optimize("no-stack-protector,fast-math")
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef pair<ll, ll> pll;
#define debug(x) cerr<<#x<<'='<<(x)<<endl;
#define debugp(x) cerr<<#x<<"= {"<<(x.first)<<", "<<(x.second)<<"}"<<endl;
#define debug2(x, y) cerr<<"{"<<#x<<", "<<#y<<"} = {"<<(x)<<", "<<(y)<<"}"<<endl;
#define debugv(v) {cerr<<#v<<" : ";for (auto x:v) cerr<<x<<' ';cerr<<endl;}
#define all(x) x.begin(), x.end()
#define pb push_back
#define kill(x) return cout<<x<<'\n', 0;
const ld eps=1e-7;
const int inf=1000000010;
const ll INF=10000000000000010LL;
const int mod=1000000007;
const int SQ=180, MAXN=180*30000;
int n, m, k, u, v, x, y, t, a, b, ans;
int B[MAXN], P[MAXN];
int dist[MAXN];
bool mark[MAXN];
vector<pii> G[MAXN];
priority_queue<pii, vector<pii>, greater<pii>> pq;
inline void addedge(int v1, int j1, int v2, int j2, int w){
G[v1*SQ+j1].pb({v2*SQ+j2, w});
}
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
memset(dist, 63, sizeof(dist));
cin>>n>>m;
for (int i=0; i<n; i++) for (int j=1; j<SQ; j++){
if (i>=j) addedge(i, j, i-j, j, 1);
if (i+j<n) addedge(i, j, i+j, j, 1);
addedge(i, j, i, 0, 0);
}
for (int i=0; i<m; i++){
cin>>B[i]>>P[i];
if (P[i]<SQ) G[B[i]*SQ].pb({B[i]*SQ+P[i], 0});
else{
for (int j=1; B[i]-j*SQ>=0; j++) G[B[i]*SQ].pb({(B[i]-j*SQ)*SQ, j});
for (int j=1; B[i]+j*SQ<n; j++) G[B[i]*SQ].pb({(B[i]+j*SQ)*SQ, j});
}
}
pq.push({dist[B[0]*SQ]=0, B[0]*SQ});
while (pq.size()){
int v=pq.top().second;
pq.pop();
if (mark[v]) continue ;
mark[v]=1;
for (pii p:G[v]) if (dist[p.first]>dist[v]+p.second)
pq.push({dist[p.first]=dist[v]+p.second, p.first});
}
ans=dist[SQ*B[1]];
if (ans>=inf) ans=-1;
cout<<ans<<'\n';
return 0;
}
