#include <bits/stdc++.h>
using namespace std;
#define int long long
#define debug(x) cerr<<#x<<" is "<<x<<endl;
signed main(){
	ios_base::sync_with_stdio(0);cin.tie(0);
	int n,t;cin>>n>>t;
	int a[n];
	for(int i=0;i<n;i++){
		cin>>a[i];
		a[i]%=t;
	}
	sort(a,a+n);
	int m=a[0]+t-a[n-1];
	for(int i=1;i<n;i++){
		m=max(m,a[i]-a[i-1]);
	}
	//debug(m)
	cout<<(t-m+1)/2<<'\n';
}
	
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