#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+10, LG=20;
int n, m;
pair<int, int> a[N];
int jump[LG][N];
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m;
vector<int> v;
for (int i=1; i<=n; ++i){
cin >> a[i].first >> a[i].second;
v.push_back(a[i].first);
v.push_back(a[i].second);
v.push_back(a[i].first+m);
v.push_back(a[i].second+m);
if (a[i].first>a[i].second) a[i].second+=m;
}
sort(v.begin(), v.end()); v.resize(unique(v.begin(), v.end())-v.begin());
int _m=m;
m=v.size();
for (int i=0; i<m; ++i) jump[0][i]=i;
for (int i=1; i<=n; ++i){
int l=lower_bound(v.begin(), v.end(), a[i].first)-v.begin();
jump[0][l]=max<int>(jump[0][l], lower_bound(v.begin(), v.end(), a[i].second)-v.begin());
}
for (int i=1; i<m; ++i) jump[0][i]=max(jump[0][i], jump[0][i-1]);
for (int i=1; i<LG; ++i) for (int j=0; j<m; ++j) jump[i][j]=jump[i-1][jump[i-1][j]];
int ans=1e9;
for (int i=1; i<=n; ++i){
int l=lower_bound(v.begin(), v.end(), a[i].first)-v.begin();
int r=lower_bound(v.begin(), v.end(), a[i].first+_m)-v.begin();
int cur=0;
for (int j=LG-1; j>=0; --j) if (jump[j][l]<r){
cur+=1<<j;
l=jump[j][l];
}
ans=min(ans, cur+1);
}
cout << (ans>n?-1:ans) << '\n';
return 0;
}
