#include <iostream>
 
using namespace std;
 
int main(){
    int n;  cin >> n;
    int cnt=0;
    int *a = new int [n];
    for(int i=1;i<=n;i++){
        cin >> a[i];
    }
    for(int i=3;i<=n;i++){
        if(a[i]==0){
            if(a[i-1]==1 && a[i+1]==1){
            	cnt++;
                i+=2;
            }
         }
    }
    cout << cnt <<endl;
    delete[] a;
    return 0;
}