C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
using namespace std;
int main(){
	int w,n,a[100000];
	cin>>w>>n;
	for(int i=1;i<=n;i++)
		cin>>a[i];
	sort(a+1,a+n+1);
	int i=1,j=n,ans=0;
	while(i<=j){
		if(a[i]+a[j]<=w){
			ans++;
			i++;
			j--;
		}
		if(a[i]+a[j]>w){
			ans++;
			j--;
		}
	}
	cout<<ans;
	return 0;
}