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

int n,k,ans,x;
int min1,max1;
int main()
{
	freopen("teststr.in","r",stdin);
	freopen("teststr.out","w",stdout);
	
	cin>>n>>k;
	ans=n;
	min1=n; max1=0;
	for (int i=0; i<k; i++)
	{
		cin>>x;
		if (x==0) continue;
		if (x>max1) max1=x;
		if (x<min1) min1=x;
	}
	ans=min1;
	if (n-max1<ans) ans=n-max1;
	if (ans==0)
		cout<<n<<endl;
	else
		cout<<ans<<endl;
	return 0;
}