#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 N=2e3+3;
int const M=3e4+3;
int b[M];
int p[M];
int d[N][N];
int const INF=1e9;
bitset<N> vis[N];
vi doges[N];
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]].pb(i);
}
F0R(i,n)
{
fill(d[i],d[i]+n,INF);
}
queue<pii>q;
q.push({b[0],p[0]});
d[b[0]][p[0]]=0;
vis[b[0]][p[0]]=1;
while(!q.empty())
{
int pos = q.front().fi;
int pw = q.front().se;
q.pop();
vector<pair<pii,int>> adj;
trav(doge,doges[pos])
{
adj.pb({{pos,p[doge]},0});
}
adj.pb({{pos,0},0});
int nxt=pos+pw;
if(nxt<n)
{
adj.pb({{nxt,pw},1});
}
nxt=pos-pw;
if(nxt>=0)
{
adj.pb({{nxt,pw},1});
}
trav(e,adj)
{
if(vis[e.fi.fi][e.fi.se])continue;
vis[e.fi.fi][e.fi.se]=1;
d[e.fi.fi][e.fi.se]=d[pos][pw]+e.se;
q.push(e.fi);
}
}
if(d[b[1]][p[1]]==INF)cout<<-1<<'\n';
else cout<<d[b[1]][p[1]]<<'\n';
return 0;
}
