// @daryn botbaev
# include <bits/stdc++.h>
# define ll long long
# define endl '\n'
# define task "lightson"
using namespace std;
const int INF=1e9+7;
const int MAXN=1e5+7;
int n,c,a[100005],b[100005],d[100005],mn=99999999,mx=-99;
int dist(int a,int b){
    return min(abs(a-b),c-abs(a-b));
}
int main () {
# ifdef Machine
freopen(task".in","r",stdin);
freopen(task".out","w",stdout);
# endif // Machine
cin>>n>>c;
for(int i=1;i<=n;++i){
    cin>>a[i]>>b[i];
}
for(int i=1;i<=n;++i){
    for(int j=1;j<=n;++j){
        if(i!=j){
            int q=dist(a[i],a[j]);
            int w=dist(a[i],b[i]);
            int e=dist(a[i],b[j]);
            int r=dist(b[i],b[j]);
            int t=dist(a[j],b[i]);
            int y=dist(a[j],b[j]);
            mn=min(min(min(q,min(w,e)),r),t);
            mn=min(mn,y);
            mx=max(mx,mn);
        }
    }
}
cout<<mx;
return 0;
}