/*input
5 3
0 2
1 1
4 1
*/
//#include "dreaming.h"
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#pragma GCC optimize("unroll-loops,no-stack-protector")
//order_of_key #of elements less than x
// find_by_order kth element
#define ll long long
#define ld long double
#define pii pair<int,int>
#define piii pair<int,pii>
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;
#define f first
#define s second
#define pb push_back
#define REP(i,n) for(int i=0;i<n;i++)
#define REP1(i,n) for(int i=1;i<=n;i++)
#define FILL(n,x) memset(n,x,sizeof(n))
#define ALL(_a) _a.begin(),_a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define MP make_pair
const ll INF64=4e12;
const int INF=0x3f3f3f3f;
const ll MOD=1e9+7;
const ld PI=acos(-1);
const ld eps=1e-9;
#define lowb(x) x&(-x)
#define MNTO(x,y) x=min(x,(__typeof__(x))y)
#define MXTO(x,y) x=max(x,(__typeof__(x))y)
ll mult(ll a,ll b){
return ((a%MOD)*(b%MOD))%MOD;
}
ll mypow(ll a,ll b){
if(b<=0) return 1;
ll res=1LL;
while(b){
if(b&1){
res=(res*a)%MOD;
}
a=(a*a)%MOD;
b>>=1;
}
return res;
}
const ll maxn=3e4+5;
const ll maxlg=__lg(maxn)+2;
vector<pii> v[maxn][205];
int b[maxn],p[maxn];
map<pii,int> dist;
vector<int> pos[maxn];
int main(){
ios::sync_with_stdio(false),cin.tie(0);
int n,m;
cin>>n>>m;
REP(i,m){
cin>>b[i]>>p[i];
pos[b[i]].pb(p[i]);
}
int blk=200;
priority_queue<piii,vector<piii>,greater<piii>> pq;
REP(i,n){
REP(j,blk+1){
dist[{i,j}]=INF;
}
}
pq.push({0,{b[0],p[0]}});
dist[{b[0],p[0]}]=0;
int ans=INF;
while(!pq.empty()){
pii z=pq.top().s;
//cout<<z.f<<' '<<z.s<<'\n';
if(sz(pq)>10) break;
pq.pop();
if(z.f==b[1]) MNTO(ans,dist[z]);
if(z.s==0){
for(int x:pos[z.f]){
if(x>blk){
for(int j=z.f%x;j<n;j+=x){
pii node={j,0};
if(dist[node]>dist[z]+abs((z.f/x)-(j/x))){
dist[node]=dist[z]+abs((z.f/x)-(j/x));
pq.push({dist[node],node});
}
}
}
else{
for(int j=z.f-x;j<=z.f+x;j+=2*x){
if(j<0 or j>n) continue;
pii node={j,x};
if(dist[node]>dist[z]+1){
dist[node]=dist[z]+1;
pq.push({dist[node],node});
}
}
}
}
}
else if(z.s<=blk){
for(int j=z.f-z.s;j<=z.f+z.s;j+=2*z.s){
if(j<0 or j>n) continue;
for(int k=0;k<=z.s;k+=z.s){
pii node={j,k};
if(dist[node]>dist[z]+1){
dist[node]=dist[z]+1;
pq.push({dist[node],node});
}
}
}
}
else{
assert(false);
}
}
if(ans==INF) cout<<-1;
else cout<<ans;
}
