#include <bits/stdc++.h>
#define int long long
using namespace std;
const int K = 20;
int INF = 1LL<<60;
signed main() {
#ifndef ONLINE_JUDGE
// for getting input from input.txt
//freopen("input.txt", "r", stdin);
// for writing output to output.txt
//freopen("output.txt", "w", stdout);
#endif
#ifdef ONLINE_JUDGE
ios_base::sync_with_stdio(false);
cin.tie(NULL);
#endif //fast IO
int n,m; cin >> n >> m;
vector<pair<int,int>> segments;
vector<int> coords;
//coords.push_back(0); //0 to m-1
map<int,int> rev;
for (int i=0; i<n; i++) {
int l,r; cin >> l >> r;
if (l<r) {
segments.push_back({l,r});
coords.push_back(l);
coords.push_back(r);
segments.push_back({m+l,m+r});
coords.push_back(m+l);
coords.push_back(m+r);
}
else {
segments.push_back({l,m+r});
coords.push_back(l);
coords.push_back(m+r);
}
}
int n0 = segments.size();
sort(coords.begin(), coords.end());
coords.erase(unique(coords.begin(), coords.end()),coords.end());
int k = coords.size();
for (int i=0; i<k; i++) {
rev[coords[i]] = i;
}
for (int i=0; i<n0; i++) {
segments[i] = {rev[segments[i].first],rev[segments[i].second]};
}
vector<int> go(k,-1);
multiset<int> ends;
set<int> indices;
sort(segments.begin(), segments.end());
int pointer = 0;
int lpointer = 0;
for (int i=0; i<k; i++) {
while (pointer<n0 && segments[pointer].first==i) {
indices.insert(pointer);
ends.insert(segments[pointer].second);
pointer++;
}
if (!ends.size()) {
go[i] = -1;
}
else {
go[i] = *ends.rbegin();
}
while (lpointer<n0 && segments[lpointer].second==i) {
indices.erase(lpointer);
ends.erase(ends.find(segments[lpointer].second));
lpointer++;
}
}
for (int i=0; i<k; i++) {
//cout << i <<" " << coords[i] <<" " << go[i] << " " << coords[go[i]] << endl;
}
vector<vector<int>> lift(k,vector<int>(K,-1));
for (int i=0; i<k; i++) {
lift[i][0] = go[i];
}
for (int p=1; p<K; p++) {
for (int i=0; i<k; i++) {
lift[i][p] = lift[lift[i][p-1]][p-1];
}
}
int mians = INF;
for (int i=0; i<k; i++) {
int cur = i;
int ct = 0;
for (int p=K-1; p>=0; p--) {
if (coords[lift[cur][p]]<coords[i]+m) {
cur = lift[cur][p];
ct+=1<<p;
}
}
int p = 0;
if (coords[lift[cur][p]]<=coords[i]+m) {
cur = lift[cur][p];
ct++;
}
if (coords[cur]>=coords[i]+m) {
mians=min(mians,ct);
}
}
if (mians<INF) cout << mians << endl;
else cout << -1 << endl;
}
