#include<iostream>
#include<vector>
#include<queue>
#include<stack>
#include<algorithm>
#include<limits.h>
#include<math.h>
#include<map>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<iomanip>
#include<cstring>
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
using namespace std;
//const int MOD=1e9+7;
//typedef pair<ll,ll>Point;
//typedef pair<ll,ll>Line;
//#define x first
//#define y second
int n,m;
int pos[30000];
int gap[30000];
vector<int>gaps[30000];
int dist[30000];
void solve(){
for(int i=0;i<30000;i++)
dist[i]=1000000;
cin>>n>>m;
for(int i=0;i<m;i++){
cin>>pos[i]>>gap[i];
gaps[pos[i]].push_back(gap[i]);
}
dist[pos[0]]=0;
queue<int>q;
q.push(pos[0]);
while(!q.empty()){
int pos=q.front();
q.pop();
for(int g:gaps[pos]){
for(int i=1;pos+g*i<n;i++){
if(dist[pos+g*i]>dist[pos]+i){
dist[pos+g*i]=dist[pos]+i;
q.push(pos+g*i);
}
}
for(int i=1;pos-g*i>=0;i++){
if(dist[pos-g*i]>dist[pos]+i){
dist[pos-g*i]=dist[pos]+i;
q.push(pos-g*i);
}
}
}
}
int res=dist[pos[1]];
if(res==1000000)
res=-1;
cout<<res<<"\n";
}
int main(){
ios::sync_with_stdio(false);cout.tie(NULL);cin.tie(NULL);
//freopen("bank.in","r",stdin);freopen("bank.out","w",stdout);
int t=1;//cin>>t;
while(t--)solve();
return 0;
}
/*
5 3
0 2
1 1
4 1
*/
