/*************************************
* author: marvinthang *
* created: 06.05.2024 15:52:23 *
*************************************/
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define left ___left
#define right ___right
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)
#define MASK(i) (1LL << (i))
#define BIT(x, i) ((x) >> (i) & 1)
#define __builtin_popcount __builtin_popcountll
#define ALL(v) (v).begin(), (v).end()
#define REP(i, n) for (int i = 0, _n = (n); i < _n; ++i)
#define REPD(i, n) for (int i = (n); i-- > 0; )
#define FOR(i, a, b) for (int i = (a), _b = (b); i < _b; ++i)
#define FORD(i, b, a) for (int i = (b), _a = (a); --i >= _a; )
#define FORE(i, a, b) for (int i = (a), _b = (b); i <= _b; ++i)
#define FORDE(i, b, a) for (int i = (b), _a = (a); i >= _a; --i)
#define scan_op(...) istream & operator >> (istream &in, __VA_ARGS__ &u)
#define print_op(...) ostream & operator << (ostream &out, const __VA_ARGS__ &u)
#ifdef LOCAL
#include "debug.h"
#else
#define file(name) if (fopen(name".inp", "r")) { freopen(name".inp", "r", stdin); freopen(name".out", "w", stdout); }
#define DB(...) 23
#define db(...) 23
#define debug(...) 23
#endif
template <class U, class V> scan_op(pair <U, V>) { return in >> u.first >> u.second; }
template <class T> scan_op(vector <T>) { for (size_t i = 0; i < u.size(); ++i) in >> u[i]; return in; }
template <class U, class V> print_op(pair <U, V>) { return out << '(' << u.first << ", " << u.second << ')'; }
template <size_t i, class T> ostream & print_tuple_utils(ostream &out, const T &tup) { if constexpr(i == tuple_size<T>::value) return out << ")"; else return print_tuple_utils<i + 1, T>(out << (i ? ", " : "(") << get<i>(tup), tup); }
template <class ...U> print_op(tuple<U...>) { return print_tuple_utils<0, tuple<U...>>(out, u); }
template <class Con, class = decltype(begin(declval<Con>()))> typename enable_if <!is_same<Con, string>::value, ostream&>::type operator << (ostream &out, const Con &con) { out << '{'; for (__typeof(con.begin()) it = con.begin(); it != con.end(); ++it) out << (it == con.begin() ? "" : ", ") << *it; return out << '}'; }
// end of template
const int LOG = 19;
void process(void) {
int n, m; cin >> n >> m;
vector <pair <int, int>> a(n); cin >> a;
vector <int> c;
REP(i, n) {
c.push_back(a[i].fi);
c.push_back(a[i].se);
}
sort(ALL(c));
c.erase(unique(ALL(c)), c.end());
REP(i, n) {
a[i].fi = lower_bound(ALL(c), a[i].fi) - c.begin();
a[i].se = lower_bound(ALL(c), a[i].se) - c.begin();
if (a[i].fi > a[i].se) a[i].se += c.size();
a.emplace_back(a[i].fi + c.size(), a[i].se + c.size());
}
n += n;
sort(ALL(a));
vector <array <pair <int, int>, LOG>> ma(n);
REP(i, n) ma[i][0] = pair{a[i].se, i};
FOR(k, 1, LOG) REP(i, n - MASK(k) + 1) ma[i][k] = max(ma[i][k - 1], ma[i + MASK(k - 1)][k - 1]);
auto get_max = [&] (int l, int r) {
if (l > r) return n;
int h = __lg(r - l + 1);
return max(ma[l][h], ma[r - MASK(h) + 1][h]).se;
};
vector <array <int, LOG>> nxt(n + 1);
nxt[n][0] = n;
REP(i, n) {
int pos = lower_bound(i + ALL(a), pair{a[i].se + 1, -1}) - a.begin() - 1;
nxt[i][0] = get_max(i + 1, pos);
}
FOR(k, 1, LOG) REP(i, n + 1) nxt[i][k] = nxt[nxt[i][k - 1]][k - 1];
int res = n + 1;
REP(i, n / 2) {
int cur = 0, u = i;
REPD(j, LOG) if (nxt[u][j] < n && a[nxt[u][j]].se < a[i].fi + c.size()) {
u = nxt[u][j];
cur ^= MASK(j);
}
if (nxt[u][0] < n) res = min(res, cur + 2);
}
cout << (res == n + 1 ? -1 : res) << '\n';
}
int main(void) {
ios_base::sync_with_stdio(false); cin.tie(nullptr); // cout.tie(nullptr);
file("fire");
// int t; cin >> t; while (t--)
process();
// cerr << "Time elapsed: " << TIME << " s.\n";
return (0^0);
}
