//~~~~~~~~~~~~~MJ®™~~~~~~~~~~~~~
#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#define rep1(n) for(ll i=0; i<(ll)(n); ++i)
#define rep2(i,n) for(ll i=0; i<(ll)(n); ++i)
#define rep3(i,a,b) for(ll i=(ll)(a); i<(ll)(b); ++i)
#define rep4(i,a,b,c) for(ll i=(ll)(a); i<(ll)(b); i+=(c))
#define cut4(a,b,c,d,e,...) e
#define rep(...) cut4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#define per1(n) for(ll i=((ll)n)-1; i>=0; --i)
#define per2(i,n) for(ll i=((ll)n)-1; i>=0; --i)
#define per3(i,a,b) for(ll i=((ll)a)-1; i>=(ll)(b); --i)
#define per4(i,a,b,c) for(ll i=((ll)a)-1; i>=(ll)(b); i-=(c))
#define per(...) cut4(__VA_ARGS__,per4,per3,per2,per1)(__VA_ARGS__)
#define ll long long
#define ln cout<<endl
#define int long long
#define Code ios_base::sync_with_stdio(0);
#define by cin.tie(NULL);
#define Hayan cout.tie(NULL);
#define append push_back
#define all(x) (x).begin(),(x).end()
#define allr(x) (x).rbegin(),(x).rend()
#define vi vector<int>
#define ret(x) {cout<<x;return;}
#define ui map<int,int>
#define pi pair<int,int>
#define ff first
#define ss second
using namespace std;
template <typename T> using v = vector<T>;
const int INF = 1e18, MOD = 1e9+7, N = 2e5+7;
void solve()
{
int n, k, e, m, ans = INF;
cin >> n >> m;
vector<pi> a(n);
rep(n)
{
cin >> a[i].ff >> a[i].ss;
if (a[i].ff>a[i].ss)a[i].ss+=m;
a[i].ff+=m;
a[i].ss+=m;
a.append({a[i].ff+m,a[i].ss+m});
a.append({a[i].ff-m,a[i].ss-m});
}
sort(all(a));
int mn,mx;
n*=3;
rep(n)
{
int op =1;
mn=a[i].ff;
mx=a[i].ss;
int j=i+1;
int k=0;
set<int> ax;
ax.insert(mx);
while (true)
{
while(j<n and a[j].ff <= mx)
{
ax.insert(a[j].ss);
j++;
}
if (mx<(*ax.rbegin()))
{
op++;
mx=*ax.rbegin();
if (mx-mn>=m)
{
ans=min(ans,op);
break;
}
}
else break;
}
// cout <<mx<<" " <<mn<<endl;
}
cout << (ans==INF ? -1: ans);
// cout << a.size();
// for (auto i: a){cout << i << " ";}
}
signed main(){
Code by Hayan
int ans=1;
//cout<<setprecision(1000);
// cin>>ans;
rep(ans){
// cout << "Case #" << i+1 << ": ";
solve();ln;}}
