#include "bits/stdc++.h"
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define MAXN 100009
#define INF 1000000007
#define mp(x,y) make_pair(x,y)
#define all(v) v.begin(),v.end()
#define pb(x) push_back(x)
#define wr cout<<"----------------"<<endl;
#define ppb() pop_back()
#define tr(ii,c) for(__typeof((c).begin()) ii=(c).begin();ii!=(c).end();ii++)
#define ff first
#define ss second
#define my_little_dodge 46
#define debug(x) cerr<< #x <<" = "<< x<<endl;
using namespace std;
typedef long long ll;
typedef pair<int,int> PII;
template<class T>bool umin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool umax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
PII arr[MAXN];
int dis[MAXN];
vector<PII>adj[MAXN];
int s,t;
int main(){
//~ freopen("file.in", "r", stdin);
int n,m;
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++){
int u,v;
scanf("%d%d",&u,&v);
if(i==1)
s=u;
if(i==2)
t=u;
arr[i]=mp(u,v);
}
sort(arr+1,arr+m+1);
for(int i=m;i>=1;i--){
int cur=1;
int u=arr[i].ff,v=arr[i].ss;
for(int j=u+v;j<n;j+=v){
adj[u].pb(mp(j,cur));
//might be break condition
cur++;
}
}
for(int i=1;i<=m;i++){
int cur=1;
int u=arr[i].ff,v=arr[i].ss;
for(int j=u-v;j>=0;j-=v){
adj[u].pb(mp(j,cur));
//might be break condition
cur++;
}
}
priority_queue<pair<int,int> >q;
q.push(mp(0,s));
for(int i=0;i<n;i++)
dis[i]=INF;
dis[s]=0;
while(!q.empty()){
int val=-q.top().ff;
int nd=q.top().ss;
q.pop();
if(dis[nd]!=val)
continue;
tr(it,adj[nd])
if(umin(dis[it->ff],dis[nd]+it->ss))
q.push(mp(-dis[it->ff],it->ff));
}
if(dis[t]==INF)
dis[t]=-1;
printf("%d\n",dis[t]);
return 0;
}
