#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 fi first
#define se second
int const M=3e4+3;
int b[M],p[M];
int const N=3e4+3;
bitset<N> vis;
int d[N];
int const INF=1e9;
unordered_set<int> doges[N];
unordered_set<int> visited;
set<int> skills[N];
int far[N];
bitset<M> lefded;
bitset<M> rigded;
// subtask 1,2,3,4
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int n,m;
cin>>n>>m;
F0R(i,m)
{
cin>>b[i]>>p[i];
doges[b[i]].insert(i);
skills[b[i]].insert(p[i]);
}
fill(d,d+n,INF);
d[b[0]]=0;
fill(far,far+n,0);
visited.insert(b[0]);
vis[b[0]]=1;
while(!visited.empty())
{
vi dead;
vi nw;
trav(place,visited)
{
++far[place];
vi rem;
trav(doge,doges[place])
{
int nxt = place + p[doge]*far[place];
bool ok=0;
if(nxt<n)
{
ok=1;
if(!vis[nxt])
{
vis[nxt]=1;
nw.pb(nxt);
d[nxt]=d[place]+far[place];
}
if(skills[nxt].find(b[doge])!=skills[nxt].end())
{
rigded[doge]=1;
}
}
nxt = place - p[doge]*far[place];
if(nxt>=0)
{
ok=1;
if(!vis[nxt])
{
vis[nxt]=1;
nw.pb(nxt);
d[nxt]=d[place]+far[place];
}
if(skills[nxt].find(b[doge])!=skills[nxt].end())
{
lefded[doge]=1;
}
}
if(!ok||(lefded[doge]&&rigded[doge]))
{
rem.pb(doge);
}
}
trav(doge,rem)
{
doges[place].erase(doge);
}
if(doges[place].empty())
{
dead.pb(place);
}
}
trav(k,dead)
{
visited.erase(k);
}
trav(k,nw)
{
visited.insert(k);
}
}
if(d[b[1]]==INF)cout<<-1<<'\n';
else cout<<d[b[1]]<<'\n';
return 0;
}
