#include <bits/stdc++.h>
#define F first
#define S second
#define pb push_back
#define ppb pop_back
#define ep insert
#define endl '\n'
#define elif else if
#define pow pwr
#define sqrt sqrtt
//#define int long long
#define ll long long
#define y1 YONE
typedef unsigned long long ull;
using namespace std;
const int N=2e5+5;
int L[N],R[N],n,m;
int32_t main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
cin>>n>>m;int ans=INT_MAX;
for (int i=1;i<=n;i++) cin>>L[i]>>R[i];
for (int k=1;k<=n;k++){
int x=L[k];
//cout<<x<<endl;
vector<pair<int,int>> a;
vector<int> p(n);
for (int i=1;i<=n;i++){
int l=(L[i]-x+m)%m,r=(R[i]-x+m)%m;
//cout<<l<<' '<<r<<endl;
if (l>r) r=m-1;
a.pb({l,r});
}
sort(a.begin(),a.end());
/*for (auto u:a) cout<<u.F<<' ';cout<<endl;
for (auto u:a) cout<<u.S<<' ';cout<<endl;
cout<<endl;*/
p[0]=a[0].S;
for (int i=1;i<n;i++) p[i]=max(p[i-1],a[i].S);
int cur=0,ansx=0;
while (cur<m-1){
int l=0,r=n-1,mid,idx=-1;
while (l<=r){
mid=(l+r)/2;
if (a[mid].F<=cur){
idx=mid;
l=mid+1;
}else r=mid-1;
}
if (idx==-1){
ansx=INT_MAX;
break;
}
if (p[idx]==cur){
ansx=INT_MAX;
break;
}
ansx++;
cur=p[idx];
}
ans=min(ans,ansx);
}
if (ans==INT_MAX) ans=-1;
cout<<ans<<endl;
return 0;
}
