#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ll long long
#define ld long double
#define all(v) v.begin(),v.end()
#define ar array
const int M = 1e6;
const int N = 4e5 + 20;
const int LOG = 61;
const int INF = 1e18;
const int MOD = 1e9 + 7;
const ld EPS = 1e-9;
inline void mm(int &x){x = (x % MOD + MOD) % MOD;}
inline void chmin(int &x, int y){x = min(x, y);}
inline void chmax(int &x, int y){x = max(x, y);}
#pragma GCC optimize("unroll-loops,O3")
void orz(){
int n, m;
cin>>n>>m;
vector<int> g[n];
int s = -1, e = -1;
while(m--){
int a, b;
cin>>a>>b;
if(s == -1)s = a;
else if(e == -1)e = a;
if(b < n)g[a].push_back(b);
}
deque<ar<int, 2> > q;
int dst[n][n];
memset(dst, -1, sizeof dst);
dst[s][0] = 0;
q.push_back({s, 0});
while(q.size()){
auto [x, j] = q.front();
q.pop_front();
//cout<<x<<" "<<j<<'\n';
for(auto u: g[x]){
if(dst[x][u] == -1){
dst[x][u] = dst[x][j];
q.push_front({x, u});
}
}
if(!j)continue;
for(auto y: {x - j, x + j}){
if(y < 0 || y >= n)continue;
if(dst[y][j] == -1){
dst[y][j] = dst[x][j] + 1;
q.push_back({y, j});
}
}
}
int ans = INF;
for(int i = 0;i < n;i++){
if(dst[e][i] != -1)ans = min(ans, dst[e][i]);
}
if(ans >= INF)ans = -1;
cout<<ans;
}
signed main(){ios_base::sync_with_stdio(false);cin.tie(0);
int t = 1;
//cin>>t;
while(t--)orz();
}
