C++ :

#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,l,i,x,ma=0;
	//freopen("ant.in","r",stdin);
	//freopen("ant.out","w",stdout);
	cin>>n;
	cin>>l;
	for(i=1;i<=n;++i){
		scanf("%d",&x);
		x=min(x,l-x);
		ma=max(ma,x);
	}
	cout<<ma;
}