#include<bits/stdc++.h>
// #include <ext/pb_ds/tree_policy.hpp>
// #include <ext/pb_ds/assoc_container.hpp>
using namespace std;
#define int long long
const int N = 2e5 + 5;
const int mod = 1e9 + 7;
const int mod1 = 998244353;
const int LG = 20;
// #define s(i) (*s.find_by_order(i)) // Warning : Read this line.
set<pair<int,int>> S;
int power(int b,int e){
if(e<=0)return 1;
int o = power(b,e>>1);
o *= o, o %= mod1;
if(e % 2)o *= b, o %= mod1;
return o;
}
void solve(){
int n,m;cin >> n >> m;
S.clear();
for(int i = 1 ; i <= n ; i ++){
int s,e;cin >> s >> e;
S.insert({s,e});
}
set<pair<int,int>> T;
for(auto a : S)
if(a.first > a.second)T.insert(a);
for(auto a : T){
S.erase(a);
S.insert({a.first, a.second + m});
}
T.clear();
for(auto a : S)
for(auto b : S){
if(a==b)continue;
if(a.first <= b.first and b.second <= a.second)
T.insert(b);
}
for(auto a : T)
S.erase(a);
vector<pair<int,int>> g;
for(auto a : S){
g.push_back(a);
if(a.second + m < 2 * m)g.push_back({a.first + m, a.second + m});
}
vector<int> next(3 * n);
int sz = g.size();
for(int i = 0 ; i < sz ; i ++){
int how=0,ind=-1;
for(int j = 0 ; j < sz ; j ++){
if(i==j)continue;
if(g[j].second <= g[i].second)continue;
if(g[j].second - g[i].second > how and g[j].first <= g[i].second)
ind = j, how = g[j].second - g[i].second;
}
next[i] = ind;
}
// for(auto i : g){
// cout << i.first << ' ' << i.second << '\n';
// }
int tot = 1e18;
for(int i = 0 ; i < sz ; i ++){
int ans = 1;
int d = g[i].second-g[i].first;
int r = i;
while(d<m){
int p = r;
r = next[r];
if(r == -1){ans=-1;break;}
d += g[r].second - g[p].second;
ans ++;
}
if(ans > -1)
tot = min(tot, ans);
}
if(tot == 1e18)tot = -1;
cout << tot << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
int t = 1;
// cin >> t;
while(t --){
solve();
}
}
