#include<iostream>
using namespace std;
string s;
int N;
int ans;
main()
{
	cin>>N>>s;
	int c1=0,c9=0;
	for(int i=0;i<N;i++)
	{
		if(s[i]=='1')
		{
			if(c9>=2)
			{
				c9-=2;
				ans++;
			}
			else c1++;
		}
		else if(s[i]=='9')
		{
			if(c1>=1)
			{
				c1-=1;
				ans++;
			}
			else c9++;
		}
		else ans++;
	}
	ans+=c1/2;
	cout<<ans<<endl;
}