#include <bits/stdc++.h>
using namespace std;
int main()
{
	long long n , a[10000] , k , t;
	cin>>k>>n;
	for ( int i=0 ;i<n;i++)
	{
		cin>>a[i];
	}
	for ( int i=0 ; i<n;i++)
	{
 
		a[0]=a[0]+k;
		break;
	}
	for ( int i=1 ; i<n;i++)
	{
		a[i]=a[i]-k;
	}
	sort(a,a+n);
	for ( int i=0 ; i<=n;i++)
	{	
		cout<<a[n-1]-a[0];
		cout<<endl;
		break;
	}
	return 0;
}

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