C++ :

#include<bits/stdc++.h>
using namespace std;
int n,guo,bing,x;
int main()
{
	cin>>n;
	while(n--)
	{
		cin>>x;
		if(x==0) guo++;
		else bing++;
	}
	cout<<min(guo/3,bing/2);
    return 0;
}