#include <iostream>
#include <vector>

using namespace std;
vector <int> a;

int main()
{
    int N;
    cin >> N;
    for(int i=0; i<N; i++)
    {
        int x;
        cin >> x;
        a.push_back(x);
    }
    int sol = 0;
    for(int i=0;i< N;i++)
    {
        if(a[i]== 1 && a[i +1] == 1 && a[i+2] != 1) {a[i]=0;sol ++;}
        else if(a[i]==1&& a[i+1] == 1 && a[i+2] == 1) {a[i +1] = 0;sol++;}
    }
    cout << sol;
    return 0;
}