#include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
#define vec vector
#define sz(x) (int)(x).size()
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define pw(x) (1LL<<(x))
#define fast_iati ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
template <class T> bool umin(T &a,const T &b){return (a>b?a=b,1:0);}
template <class T> bool umax(T &a,const T &b){return (a<b?a=b,1:0);}
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
signed main(){
fast_iati;
int n,m;
cin>>n>>m;
vec<int>b(m),p(m);
vec<vec<int>>have(n,vec<int>());
for(int i=0;i<m;i++)
cin>>b[i]>>p[i],have[b[i]].pb(i);
priority_queue<pii,vec<pii>,greater<pii>>pq;
vec<int>dp(n,2e9);
// vec<priority_queue<pii>>kek(n,0);
pq.push({b[0],0});
dp[b[0]]=0;
while(!pq.empty()){
int v=pq.top().s;
int x=pq.top().f;
pq.pop();
if(dp[v]!=x) continue;
for(auto &u : have[v]){
int j=b[u]-p[u];
int cnt=1;
while(j>=0){
if(umin(dp[j],dp[v]+cnt))
pq.push({dp[j],j});
j-=p[u];cnt++;
}
j=b[u]+p[u];cnt=1;
while(j<n){
if(umin(dp[j],dp[v]+cnt))
pq.push({dp[j],j});
j+=p[u];cnt++;
}
}
}
cout<<(dp[b[1]]==2e9?-1:dp[b[1]]);
return 0;
}
/*
6 1 3
8 1 2 1 5 4
*/
