#include <iostream>
#include <vector>
using namespace std;
vector<int> panel;
int main()
{
    int n, x, changes = 0;
    cin>>n;
    cin>>x;
    panel.push_back(x);
    for(int i = 2; i <= n; i++)
    {
        cin>>x;
        if(x == 1 and panel[i-2] == 1)
        {
            panel.push_back(0);
            changes++;
        }
        else
            panel.push_back(x);

    }
    cout<<changes<<endl;
}