#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
const int MOD = 1000000007;
const ll INF = 1e18;
const int MX = 30001;
int N,M, st, en;
bitset<MX> oc[MX], visit;
vi sky[MX];
queue<array<int,3>> q;
void tri(int x, array<int,3> y) {
if (x < 0 || x >= N) return;
if (oc[x][y[2]]) return;
oc[x][y[2]] = 1;
q.push({y[0]+1,x,y[2]});
if (!visit[x]) {
for (int i: sky[x]) q.push({y[0]+1,x,i});
visit[x] = 1;
}
}
int main() {
ios_base::sync_with_stdio(0); cin.tie(0);
cin >> N >> M;
F0R(i,M) {
int B,P; cin >> B >> P;
sky[B].pb(P);
if (i == 0) st = B;
if (i == 1) en = B;
}
for (int i: sky[st]) {
oc[st][i] = 1;
q.push({0,st,i});
}
while (sz(q)) {
auto a = q.front(); q.pop();
if (a[1] == en) {
cout << a[0];
exit(0);
}
tri(a[1]-a[2],a);
tri(a[1]+a[2],a);
}
cout << -1;
}
/* Look for:
* the exact constraints (multiple sets are too slow for n=10^6 :( )
* special cases (n=1?)
* overflow (ll vs int?)
* array bounds
*/
