#include <iostream>
using namespace std;

int main(){
    int n;
    bool a = 0;
    bool last_a = 0;
    int numm = 0;
    cin >> n;
    for(int i = 0; i < n; ++i){
        cin >> a;
        if(last_a == true && a == true){
            ++numm;
            last_a = false;
        }else{
        last_a = a;
        }
    }
    cout << numm <<endl;

}