#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
using namespace std;
#define L(i,j,k) for(int i=(j);i<=(k);i++)
#define R(i,j,k) for(int i=(j);i>=(k);i--)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
void solve(){
int n,m;
cin>>n>>m;
vector<int> pos(m);
vector<int> pulo(m);
L(i,0,m-1)cin>>pos[i]>>pulo[i];
vector<vector<int>> amigos(n);
L(i,0,m-1){
amigos[pos[i]].push_back(i);
}
const int MOD=1e9+7;
vector<int> dp(m,MOD);
priority_queue<pair<int,int>,vector<pair<int,int>>,greater<pair<int,int>>> pq;
for(auto a:amigos[pos[0]]){
dp[a]=0;
pq.push({0,a});
}
amigos[pos[0]].clear();
while(!pq.empty()){//posso deletar os carinhas do amigo se tiver ruim
auto [d,v]=pq.top();
pq.pop();
if(d!=dp[v])continue;
if(v==1)break;
for(int i=1;pulo[v]*i+pos[v]<n;i++){
int casa=i*pulo[v]+pos[v];
for(auto a:amigos[casa]){
if(dp[a]>dp[v]+i){
dp[a]=dp[v]+i;
pq.push({dp[a],a});
}
}
}
for(int i=1;pos[v]-pulo[v]*i>=0;i++){
int casa=pos[v]-i*pulo[v];
for(auto a:amigos[casa]){
if(dp[a]>dp[v]+i){
dp[a]=dp[v]+i;
pq.push({dp[a],a});
}
}
}
}
// L(i,0,m-1)cout<<dp[i]<<" ";
cout<<((dp[1]==MOD)?-1:dp[1]);
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
solve();
}
