#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

const int maxk=10000+1;
const int maxn=1000000+1;
int a[maxk];
int f,g;
int n,k;
int main()
{
	freopen("teststr.in","r",stdin);
	freopen("teststr.out","w",stdout);
	
	cin>>n>>k;
	for (int i=1; i<=k; i++)
	{
		cin>>a[i];
		if (a[i]>f)
			f=a[i];
		else 
			g=a[i];
	}
	if (f>=n/2 && g>=n/2)
		cout<<g<<endl;
	else if(f>=n/2 && g<n/2)
	{
		if (n-f>g)
			cout<<n-f<<endl;
		else
			cout<<g<<endl;
	}
	else 
		cout<<n-f<<endl;
	return 0;
}