#include <iostream>

using namespace std;

int main()
{
    int n,a,b,nr_op=0;
    cin>>n;
    int i=2;
    cin>>a;
    while (i<=n)
    {
        cin>>b;
        if (a==1 && b==1)
        {
            nr_op++;
            b=0;
        }
        a=b;
        i++;
    }
    cout<<nr_op<<endl;
    return 0;
}