C++ :

#include<bits/stdc++.h>
using namespace std;
int n,m,x,tmp;
int main()
{
//	freopen("749.in","r",stdin); 	
//	freopen("749.out","w",stdout); 
	cin>>n>>m;
	for(int i=0;i<m;i++)
	{
		cin>>x;
		tmp+=n-x;
	}
	tmp=n-tmp;
	if(tmp>=0) cout<<tmp;
	else cout<<0;
    return 0;
}