#include <iostream>     
using namespace std;

int main()
{
	int n,i,db=0;
	int * v;


	cin >> n;

	v = (int*)calloc(n, sizeof(int));

	for (i = 0; i<n; i++)
	{
		
		cin>> v[i];

	}
	i = 1;
	while (i < n)
	{
		if (v[i - 1] && v[i]) { v[i] = 0; db++; }
		i++;
	}

	free(v);
	cout << db;
	return 0;
}