#include<stdio.h>
int main()
{
	int n;
	scanf("%d", &n);
	int i, j;
	int a[5003];
	for (i = 0; i < n; i++)
		scanf("%d", &a[i]);
	a[n] = 2;
	int count[3] = { 0,0,0 };
	for (i = 0; i < n; i++)
		count[a[i]]++;
	int cnt[5003][2], m = 0;
	cnt[0][0] = cnt[0][1] = 0;
	for (i = 0; i <= n; i++)
	{
		if (a[i] == 2)
		{
			m++;
			cnt[m][0] = cnt[m][1] = 0;
		}
		else
			cnt[m][a[i]]++;
	}
	int dp[5003][5003];
	for (i = 0; i < 5003; i++)
		for (j = 0; j < 5003; j++)
			dp[i][j] = -1;
	dp[0][0] = cnt[0][0];
	dp[0][cnt[0][0] + cnt[0][1]] = cnt[0][1];
	for (i = 1; i < m; i++)
	{
		for (j = 0; j < 5003; j++)
		{
			if (dp[i - 1][j] >= 0)
				dp[i][j] = dp[i - 1][j] + cnt[i][0];
			if (j >= cnt[i][0] + cnt[i][1])
			{
				if (dp[i - 1][j - cnt[i][0] - cnt[i][1]] >= 0)
				{
					if (dp[i][j]<0 || dp[i][j]>dp[i - 1][j - cnt[i][0] - cnt[i][1]] + cnt[i][1])
						dp[i][j] = dp[i - 1][j - cnt[i][0] - cnt[i][1]] + cnt[i][1];
				}
			}
		}
	}
	if (dp[m - 1][count[0]] > 0)
		dp[m - 1][count[0]] /= 2;
	printf("%d\n", dp[m - 1][count[0]]);
	return 0;
}