#include<bits/stdc++.h>
#pragma GCC optimize ("Ofast")
using namespace std;
#define rep(i,a,b) for(ll i = a; i<b;i++)
#define rrep(i,a,b) for(ll i = b-1; i>=a;i--)
#define trav(x,a) for(auto &x: a)
#define all(v) v.begin(),v.end()
#define sz(v) ll(v.size())
typedef long long ll;
typedef vector<ll> vl;
typedef pair<ll,ll> pll;
int main(){
cin.tie(0);
ios_base::sync_with_stdio(0);
ll n,m; cin>>n>>m;
vl s(n);
vl e(n);
rep(i,0,n) cin>>s[i]>>e[i];
vector<tuple<ll,ll,ll>> ev;
rep(i,0,n){
if(e[i]<s[i]){
e[i]+=m;
}
ev.emplace_back(s[i],0,i);
ev.emplace_back(e[i],1,i);
ev.emplace_back(s[i]+m,0,i);
ev.emplace_back(e[i]+m,1,i);
ev.emplace_back(s[i]+m*2,0,i);
ev.emplace_back(e[i]+m*2,1,i);
}
map<ll,ll> nextInterval;
sort(all(ev));
reverse(all(ev));
set<pll> best;
for(auto x: ev){
auto [t,b,idx] = x;
if(b==1){
if(sz(best)){
nextInterval[t] = max(nextInterval[t], prev(best.end())->first);
}
best.emplace(t,idx);
} else {
best.erase(make_pair(t+e[idx]-s[idx],idx));
}
}
if(sz(nextInterval)==0){
cout<<-1<<"\n";
return 0;
}
// for(auto x: nextInterval){
// cout<<x.first<<": "<<x.second<<endl;
// }
deque<ll> q;
q.push_front(nextInterval.begin()->first);
ll topScore = 1e18;
rep(i,0,10*n){
// cout<<"q:"<<endl;
// for(auto x:q){
// cout<<x<<" ";
// }
// cout<<endl;
if(q.front()-q.back()>=m){
topScore = min(topScore, sz(q)-1);
q.pop_back();
} else {
if(nextInterval.find(q.front()) == nextInterval.end()){
if(nextInterval.lower_bound(q.front())==nextInterval.end()) break;
else {
ll t = nextInterval.lower_bound(q.front())->first;
q.clear();
q.push_front(t);
}
}
q.push_front(nextInterval[q.front()]);
}
}
if(topScore==1e18){
cout<<-1<<"\n";
} else
cout<<topScore<<"\n";
}
