#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(),x.end()
#define pb push_back
#define bug(a) std::cerr << "(" << #a << ": " << a << ")\n";
using i64 = long long;
const int N=30'000;
struct cord
{
int pos,step;
bool operator <(const cord &b)const
{
if(pos==b.pos)
{
return step<b.step;
}
return pos<b.pos;
}
bool operator ==(const cord &b)const
{
return pos==b.pos && step==b.step;
}
};
struct hashPi {
size_t operator()(const cord &p) const { return 30'000*p.pos+p.step; }
};
unordered_map<cord , int , hashPi>dist;
vector<int>adj[N];
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
int n,m;
cin>>n>>m;
vector<cord>cords(m);
for(auto &c:cords)
{
cin>>c.pos>>c.step;
adj[c.pos].push_back(c.step);
}
deque<cord>q;
q.push_front(cords[0]);
vector<bool>viz(n,0);
auto insert=[&](cord nextt,int cost,bool fronted)
{
if(!dist.count(nextt) || dist[nextt]>cost)
{
dist[nextt]=cost;
if(fronted)
{
q.push_front(nextt);
}
else
{
q.push_back(nextt);
}
}
};
while(q.size())
{
cord then=q.front();
int then_cost=dist[then];
q.pop_front();
if(then.pos==cords[1].pos)
{
cout<<then_cost<<'\n';
return 0;
}
if(!viz[then.pos])
{
viz[then.pos]=true;
for(auto &c:adj[then.pos])
{
insert((cord){then.pos , c} , then_cost , true);
}
adj[then.pos].clear();
}
if(then.pos>=then.step)
{
insert({then.pos-then.step , then.step} , then_cost+1 , false);
}
if(then.pos+then.step<n)
{
insert({then.pos+then.step , then.step} , then_cost+1 , false);
}
assert(dist.size()<=10'000'000);
}
cout<<-1;
}
