#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn = 2e5;
int n, m, nxt[maxn + 1], ttime;
int ans = maxn + 5;
int vis[maxn + 1];
vector<int> value;
pair<int, int> a[maxn + 1];
struct SEGMENT_TREE{
vector<pair<int, int>> st;
inline SEGMENT_TREE(int n){
st.resize(4 * n + 4, {0, 0});
}
inline void update(int ind, int l, int r, int pos, int val, int id){
if (l == r){
st[ind] = max(st[ind], make_pair(val, id));
return;
}
int mid = (l + r) >> 1;
if (mid >= pos) update(ind << 1, l, mid, pos, val, id);
else update(ind << 1 | 1, mid + 1, r, pos, val, id);
st[ind] = max(st[ind << 1], st[ind << 1 | 1]);
return;
}
inline pair<int, int> get(int ind, int l, int r, int lb, int rb){
if (lb > rb) return {0, 0};
if (l >= lb && r <= rb) return st[ind];
int mid = (l + r) >> 1;
pair<int, int> res = {0, 0};
if (mid >= lb) res = max(res, get(ind << 1, l, mid, lb, rb));
if (mid < rb) res = max(res, get(ind << 1 | 1, mid + 1, r, lb, rb));
return res;
}
};
inline int DFS(int u){
vis[u] = ttime;
int v = nxt[u];
if (v == -1)
return maxn + 5;
if (vis[v] == 0)
return DFS(v);
if (vis[v] == ttime){
int start = v, res = 0;
while(true){
res++;
if (start == u)
break;
start = nxt[start];
}
return res;
}
return 0;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
memset(vis, 0, sizeof(vis));
cin >> n >> m;
for (int i = 1; i <= n; ++i){
cin >> a[i].first >> a[i].second;
value.push_back(a[i].first);
value.push_back(a[i].second);
}
sort(value.begin(), value.end());
value.erase(unique(value.begin(), value.end()), value.end());
SEGMENT_TREE seg(value.size());
for (int i = 1; i <= n; ++i){
int pos = lower_bound(value.begin(), value.end(), a[i].first) - value.begin();
int val = a[i].second;
if (a[i].second < a[i].first)
val += m;
// cout << pos << ' ' << val << '\n';
seg.update(1, 0, value.size() - 1, pos, val, i);
}
for (int i = 1; i <= n; ++i){
int it1 = lower_bound(value.begin(), value.end(), a[i].first) - value.begin();
int it2 = lower_bound(value.begin(), value.end(), a[i].second) - value.begin();
pair<int, int> opt = {0, 0};
if (a[i].second < a[i].first)
opt = max(seg.get(1, 0, value.size() - 1, it1 + 1, value.size() - 1), seg.get(1, 0, value.size() - 1, 0, it2));
else
opt = seg.get(1, 0, value.size() - 1, it1 + 1, it2);
if (opt.first <= a[i].second)
nxt[i] = -1;
else
nxt[i] = opt.second;
// cout << opt.first << ' ' << opt.second << '\n';
}
for (int i = 1; i <= n; ++i) if (vis[i] == 0){
++ttime;
ans = min(ans, DFS(i));
}
if (ans == maxn + 5) ans = -1;
cout << ans;
return 0;
}
