#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vpi;
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
#define pb push_back
#define rsz resize
#define sz(x) int(x.size())
#define all(x) begin(x), end(x)
#define f first
#define s second
int N,M;
int const m=3e4, n=3e4;
int B[m];
int P[m];
vi sky[n];
int dist[m];
priority_queue<pii>q;
bitset<m> pro;
void activate(int now, vpi &adjnow)
{
for(int i=0; B[now]+i*P[now]<N; ++i)
{
trav(e,sky[B[now]+i*P[now]])
{
if(pro[e])continue;
adjnow.pb({e,i});
}
}
for(int i=-1; B[now]+i*P[now]>=0; --i)
{
trav(e,sky[B[now]+i*P[now]])
{
if(pro[e])continue;
adjnow.pb({e,-i});
}
}
}
void dij(int x)
{
dist[x]=0;
q.push({0,x});
int b,w;
vpi adjnow;
while(!q.empty())
{
int a = q.top().s;
q.pop();
if(pro[a])continue;
pro[a]=1;
if(a==1)return;
adjnow.clear();
activate(a, adjnow);
trav(e, adjnow)
{
b=e.f;
w=e.s;
if(dist[a]+w<dist[b])
{
dist[b]=dist[a]+w;
q.push({-dist[b],b});
}
}
}
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
cin>>N>>M;
F0R(i,M)
{
cin>>B[i]>>P[i];
sky[B[i]].pb(i);
}
int INF=1e9;
F0R(now,M)
{
dist[now]=INF;
}
dij(0);
if(dist[1]==INF)cout<<-1<<'\n';
else cout<<dist[1]<<'\n';
return 0;
}
