#include<bits/stdc++.h>
#include<chrono>
#include<random>
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef double db;
ll gcd(ll a , ll b) {return b ? gcd(b , a % b) : a ;} // greatest common divisor (PGCD)
ll lcm(ll a , ll b) {return (a * b) / gcd(a , b);} // least common multiple (PPCM)
int dx[8] = {1, 0, 0, -1, 1, 1, -1, -1};
int dy[8] = {0, 1, -1, 0, 1, -1, -1, 1};
#define endl "\n"
#define ss second
#define ff first
#define all(x) (x).begin() , (x).end()
#define pb push_back
#define vi vector<int>
#define vii vector<pair<int,int>>
#define vl vector<ll>
#define vll vector<pair<ll,ll>>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pdd pair<double,double>
#define vdd vector<pdd>
#define speed ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define pbds tree<pii, null_type, less<pii>,rb_tree_tag, tree_order_statistics_node_update>
using namespace __gnu_pbds;
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
////////////////////Only Clear Code//////////////////////////
void usaco_problem(){
freopen("milkvisits.in", "r", stdin);
freopen("milkvisits.out", "w", stdout);
}
void init(){
#ifndef ONLINE_JUDGE
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif // ONLINE_JUDGE
}
const int mx = 305;
const int LOG = 22;
const int inf = 1e9;
const ll mod = 1e9+7;
const int sq = 320;
int n, m;
vii v;
bool used[mx];
int32_t main(){
speed;
cin >> n >> m;
set<pii> s;
int big = 0;
for(int i = 0; i < n;i++){
int a, b;cin >> a >> b;
if(a > b){
big = max(big, b);
b += m;
}
if(s.count({a, b}))continue;
s.insert({a, b});
v.pb({a, b});
}
n = s.size();
sort(all(v));
for(pii p : v){
//cout << p.ff << " " << p.ss << endl;
}
int sz = v[0].ss - v[0].ff;
//if(sz <= sqrt){
int ans = 1e9;
for(int i = 0; i < n;i++){
if(v[i].ff > big)break;
int l = v[i].ff, r = v[i].ss;
int cnt = 1;
int target = l+m;
int ind = i+1;
if(r >= target){
ans = min(ans,cnt);
continue;
}
for(int j = 0;j < n;j++){
int last = -1;
while(ind < n && v[ind].ff <= r){
last = max(last, v[ind].ss);
ind++;
}
if(last > r){
r = last;
cnt++;
}
else{
cnt = 1e9;
break;
}
if(r >= target){
break;
}
}
ans = min(ans,cnt);
}
cout << (ans == 1e9 ? -1 : ans) << endl;
//}
}
/*
NEVER GIVE UP!
DOING SMTHNG IS BETTER THAN DOING NTHNG!!!
Your Guide when stuck:
- Continue keyword only after reading the whole input
- Don't use memset with testcases
- Check for corner cases(n=1, n=0)
- Check where you declare n(Be careful of declaring it globally and in main)
*/
