#include <bits/stdc++.h>
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define debug(x) cout << "Line " << __LINE__ << ", " << #x << " is " << x << endl
#define all(x) x.begin(), x.end()
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ll long long
#define ull unsigned long long
#define pll pair<ll, ll>
#define pii pair<int, int>
#define ld long double
#define nl endl
#define tb '\t'
#define sp ' '
#define sqr(x) (x)*(x)
#define arr3 array<int, 3>
using namespace std;
const int MX=30005, MOD=1000000007, BLOCK=160, INF=2e9+7, LG=62;
const ll INFF=1000000000000000007;
const ld ERR=1e-6, pi=3.14159265358979323846;
int N, M, B[MX], P[MX];
int vis[MX][BLOCK+5], vis2[MX];
vector<int> doge[MX];
int main(){
fastio;
cin >> N >> M;
for(int i=0; i<=N; i++){
for(int j=0; j<=BLOCK; j++)
vis[i][j]=INF;
}
for(int i=0; i<M; i++){
cin >> B[i] >> P[i];
doge[B[i]].pb(P[i]);
}
priority_queue<arr3, vector<arr3>, greater<arr3>> PQ;
PQ.push({0, B[0], 0});
vis[B[0]][0]=0;
while(PQ.size()){
int cost=PQ.top()[0], idx=PQ.top()[1], p=PQ.top()[2];
PQ.pop();
if(vis[idx][p]<cost)
continue;
vis[idx][p]=cost;
if(!p){
for(int i:doge[idx]){
if(i<=BLOCK){
if(vis[idx][i]<=cost)
continue;
vis[idx][i]=cost;
PQ.push({cost, idx, i});
}else{
for(int j=1, jj; idx+i*j<N; j++){
jj=idx+i*j;
if(vis[jj][0]>cost+j){
vis[jj][0]=cost+j;
PQ.push({cost+j, jj, 0});
}
}
for(int j=1, jj; idx-i*j>=0; j++){
jj=idx-i*j;
if(vis[jj][0]>cost+j){
vis[jj][0]=cost+j;
PQ.push({cost+j, jj, 0});
}
}
}
}
}else{
for(int j=1, jj; j<=1; j++){
jj=idx+p*j;
if(jj>=N)
continue;
if(vis[jj][0]>cost+j){
vis[jj][0]=cost+j;
PQ.push({cost+j, jj, 0});
}
if(vis[jj][p]>cost+j){
vis[jj][p]=cost+j;
PQ.push({cost+j, jj, p});
}
}
for(int j=1, jj; j<=1; j++){
jj=idx-p*j;
if(jj<0)
continue;
if(vis[jj][0]>cost+j){
vis[jj][0]=cost+j;
PQ.push({cost+j, jj, 0});
}
if(vis[jj][p]>cost+j){
vis[jj][p]=cost+j;
PQ.push({cost+j, jj, p});
}
}
}
}
int ans=vis[B[1]][0];
if(ans==INF)
ans=-1;
cout << ans << nl;
}
